Develop Reference

Will "==" evaluate as "===" in JS strict mode? [duplicate]

Recently, I ran some of my JavaScript code through Crockford's JSLint, and it gave the following error:
Problem at line 1 character 1: Missing "use strict" statement.
Doing some searching, I realized that some people add "use strict"; into their JavaScript code. Once I added the statement, the error stopped appearing. Unfortunately, Google did not reveal much of the history behind this string statement. Certainly it must have something to do with how the JavaScript is interpreted by the browser, but I have no idea what the effect would be.
So what is "use strict"; all about, what does it imply, and is it still relevant?
Do any of the current browsers respond to the "use strict"; string or is it for future use?

Update for ES6 modules
Inside native ECMAScript modules (with import and export statements) and ES6 classes, strict mode is always enabled and cannot be disabled.
Original answer
This article about Javascript Strict Mode might interest you: John Resig - ECMAScript 5 Strict Mode, JSON, and More
To quote some interesting parts:
Strict Mode is a new feature in ECMAScript 5 that allows you to place a program, or a function, in a "strict" operating context. This strict context prevents certain actions from being taken and throws more exceptions.
And:
Strict mode helps out in a couple ways:
It catches some common coding bloopers, throwing exceptions.
It prevents, or throws errors, when relatively "unsafe" actions are taken (such as gaining access to the global object).
It disables features that are confusing or poorly thought out.
Also note you can apply "strict mode" to the whole file... Or you can use it only for a specific function (still quoting from John Resig's article):
// Non-strict code...
(function(){
"use strict";
// Define your library strictly...
})();
// Non-strict code...
Which might be helpful if you have to mix old and new code ;-)
So, I suppose it's a bit like the "use strict" you can use in Perl (hence the name?): it helps you make fewer errors, by detecting more things that could lead to breakages.
Strict mode is now supported by all major browsers.

It's a new feature of ECMAScript 5. John Resig wrote up a nice summary of it.
It's just a string you put in your JavaScript files (either at the top of your file or inside of a function) that looks like this:
"use strict";
Putting it in your code now shouldn't cause any problems with current browsers as it's just a string. It may cause problems with your code in the future if your code violates the pragma. For instance, if you currently have foo = "bar" without defining foo first, your code will start failing...which is a good thing in my opinion.

The statement "use strict"; instructs the browser to use the Strict mode, which is a reduced and safer feature set of JavaScript.
List of features (non-exhaustive)
Disallows global variables. (Catches missing var declarations and typos in variable names)
Silent failing assignments will throw error in strict mode (assigning NaN = 5;)
Attempts to delete undeletable properties will throw (delete Object.prototype)
Requires all property names in an object literal to be unique (var x = {x1: "1", x1: "2"})
Function parameter names must be unique (function sum (x, x) {...})
Forbids octal syntax (var x = 023; some devs assume wrongly that a preceding zero does nothing to change the number.)
Forbids the with keyword
eval in strict mode does not introduce new variables
Forbids deleting plain names (delete x;)
Forbids binding or assignment of the names eval and arguments in any form
Strict mode does not alias properties of the arguments object with the formal parameters. (e.g. in function sum (a,b) { return arguments[0] + b;} This works because arguments[0] is bound to a and so on. ) (See examples section below to understand the difference)
arguments.callee is not supported
[Ref: Strict mode, Mozilla Developer Network]
Examples:
Strict mode code doesn't alias properties of arguments objects created within it
function show( msg ){
msg = 42;
console.log( msg ); // msg === 42
console.log( arguments[0] ); // arguments === 42
}
show( "Hey" );
// In strict mode arguments[i] does not track the value of
// the corresponding named argument, nor does a named argument track the value in the corresponding arguments[i]
function showStrict( msg ){
"use strict";
msg = 42;
console.log( msg ); // msg === 42
console.log( arguments[0] ); // arguments === "Hey"
}
showStrict( "Hey" );

If people are worried about using use strict it might be worth checking out this article:
ECMAScript 5 'Strict mode' support in browsers. What does this mean?
NovoGeek.com - Krishna's weblog
It talks about browser support, but more importantly how to deal with it safely:
function isStrictMode(){
return !this;
}
/*
returns false, since 'this' refers to global object and
'!this' becomes false
*/
function isStrictMode(){
"use strict";
return !this;
}
/*
returns true, since in strict mode the keyword 'this'
does not refer to global object, unlike traditional JS.
So here, 'this' is 'undefined' and '!this' becomes true.
*/

A word of caution, all you hard-charging programmers: applying "use strict" to existing code can be hazardous! This thing is not some feel-good, happy-face sticker that you can slap on the code to make it 'better'. With the "use strict" pragma, the browser will suddenly THROW exceptions in random places that it never threw before just because at that spot you are doing something that default/loose JavaScript happily allows but strict JavaScript abhors! You may have strictness violations hiding in seldom used calls in your code that will only throw an exception when they do eventually get run - say, in the production environment that your paying customers use!
If you are going to take the plunge, it is a good idea to apply "use strict" alongside comprehensive unit tests and a strictly configured JSHint build task that will give you some confidence that there is no dark corner of your module that will blow up horribly just because you've turned on Strict Mode. Or, hey, here's another option: just don't add "use strict" to any of your legacy code, it's probably safer that way, honestly. DEFINITELY DO NOT add "use strict" to any modules you do not own or maintain, like third party modules.
I think even though it is a deadly caged animal, "use strict" can be good stuff, but you have to do it right. The best time to go strict is when your project is greenfield and you are starting from scratch. Configure JSHint/JSLint with all the warnings and options cranked up as tight as your team can stomach, get a good build/test/assert system du jour rigged like Grunt+Karma+Chai, and only THEN start marking all your new modules as "use strict". Be prepared to cure lots of niggly errors and warnings. Make sure everyone understands the gravity by configuring the build to FAIL if JSHint/JSLint produces any violations.
My project was not a greenfield project when I adopted "use strict". As a result, my IDE is full of red marks because I don't have "use strict" on half my modules, and JSHint complains about that. It's a reminder to me about what refactoring I should do in the future. My goal is to be red mark free due to all of my missing "use strict" statements, but that is years away now.

Using 'use strict'; does not suddenly make your code better.
The JavaScript strict mode is a feature in ECMAScript 5. You can enable the strict mode by declaring this in the top of your script/function.
'use strict';
When a JavaScript engine sees this directive, it will start to interpret the code in a special mode. In this mode, errors are thrown up when certain coding practices that could end up being potential bugs are detected (which is the reasoning behind the strict mode).
Consider this example:
var a = 365;
var b = 030;
In their obsession to line up the numeric literals, the developer has inadvertently initialized variable b with an octal literal. Non-strict mode will interpret this as a numeric literal with value 24 (in base 10). However, strict mode will throw an error.
For a non-exhaustive list of specialties in strict mode, see this answer.
Where should I use 'use strict';?
In my new JavaScript application: Absolutely! Strict mode can be used as a whistleblower when you are doing something stupid with your code.
In my existing JavaScript code: Probably not! If your existing JavaScript code has statements that are prohibited in strict-mode, the application will simply break. If you want strict mode, you should be prepared to debug and correct your existing code. This is why using 'use strict'; does not suddenly make your code better.
How do I use strict mode?
Insert a 'use strict'; statement on top of your script:
// File: myscript.js
'use strict';
var a = 2;
....
Note that everything in the file myscript.js will be interpreted in strict mode.
Or, insert a 'use strict'; statement on top of your function body:
function doSomething() {
'use strict';
...
}
Everything in the lexical scope of function doSomething will be interpreted in strict mode. The word lexical scope is important here. For example, if your strict code calls a function of a library that is not strict, only your code is executed in strict mode, and not the called function. See this answer for a better explanation.
What things are prohibited in strict mode?
I found a nice article describing several things that are prohibited in strict mode (note that this is not an exhaustive list):
Scope
Historically, JavaScript has been confused about how functions
are scoped. Sometimes they seem to be statically scoped, but some
features make them behave like they are dynamically scoped. This is
confusing, making programs difficult to read and understand.
Misunderstanding causes bugs. It also is a problem for performance.
Static scoping would permit variable binding to happen at compile
time, but the requirement for dynamic scope means the binding must be
deferred to runtime, which comes with a significant performance
penalty.
Strict mode requires that all variable binding be done statically.
That means that the features that previously required dynamic binding
must be eliminated or modified. Specifically, the with statement is
eliminated, and the eval function’s ability to tamper with the
environment of its caller is severely restricted.
One of the benefits of strict code is that tools like YUI Compressor
can do a better job when processing it.
Implied Global Variables
JavaScript has implied global variables. If
you do not explicitly declare a variable, a global variable is
implicitly declared for you. This makes programming easier for
beginners because they can neglect some of their basic housekeeping
chores. But it makes the management of larger programs much more
difficult and it significantly degrades reliability. So in strict
mode, implied global variables are no longer created. You should
explicitly declare all of your variables.
Global Leakage
There are a number of situations that could cause this
to be bound to the global object. For example, if you forget to
provide the new prefix when calling a constructor function, the
constructor's this will be bound unexpectedly to the global object, so
instead of initializing a new object, it will instead be silently
tampering with global variables. In these situations, strict mode will
instead bind this to undefined, which will cause the constructor to
throw an exception instead, allowing the error to be detected much
sooner.
Noisy Failure
JavaScript has always had read-only properties, but you
could not create them yourself until ES5’s Object.createProperty
function exposed that capability. If you attempted to assign a value
to a read-only property, it would fail silently. The assignment would
not change the property’s value, but your program would proceed as
though it had. This is an integrity hazard that can cause programs to
go into an inconsistent state. In strict mode, attempting to change a
read-only property will throw an exception.
Octal
The octal (or base 8) representation of numbers was extremely
useful when doing machine-level programming on machines whose word
sizes were a multiple of 3. You needed octal when working with the CDC
6600 mainframe, which had a word size of 60 bits. If you could read
octal, you could look at a word as 20 digits. Two digits represented
the op code, and one digit identified one of 8 registers. During the
slow transition from machine codes to high level languages, it was
thought to be useful to provide octal forms in programming languages.
In C, an extremely unfortunate representation of octalness was
selected: Leading zero. So in C, 0100 means 64, not 100, and 08 is an
error, not 8. Even more unfortunately, this anachronism has been
copied into nearly all modern languages, including JavaScript, where
it is only used to create errors. It has no other purpose. So in
strict mode, octal forms are no longer allowed.
Et cetera
The arguments pseudo array becomes a little bit more
array-like in ES5. In strict mode, it loses its callee and caller
properties. This makes it possible to pass your arguments to untrusted
code without giving up a lot of confidential context. Also, the
arguments property of functions is eliminated.
In strict mode, duplicate keys in a function literal will produce a
syntax error. A function can’t have two parameters with the same name.
A function can’t have a variable with the same name as one of its
parameters. A function can’t delete its own variables. An attempt to
delete a non-configurable property now throws an exception. Primitive
values are not implicitly wrapped.
Reserved words for future JavaScript versions
ECMAScript 5 adds a list of reserved words. If you use them as variables or arguments, strict mode will throw an error. The reserved words are:
implements, interface, let, package, private, protected, public, static, and yield
Further Reading
Strict Mode - JavaScript | MDN
Browser support for strict mode
Transitioning to strict mode

I strongly recommend every developer to start using strict mode now. There are enough browsers supporting it that strict mode will legitimately help save us from errors we didn’t even know were in your code.
Apparently, at the initial stage there will be errors we have never encountered before. To get the full benefit, we need to do proper testing after switching to strict mode to make sure we have caught everything. Definitely we don’t just throw use strict in our code and assume there are no errors. So the churn is that it’s time to start using this incredibly useful language feature to write better code.
For example,
var person = {
name : 'xyz',
position : 'abc',
fullname : function () { "use strict"; return this.name; }
};
JSLint is a debugger written by Douglas Crockford. Simply paste in your script, and it’ll quickly scan for any noticeable issues and errors in your code.

I would like to offer a somewhat more founded answer complementing the other answers. I was hoping to edit the most popular answer, but failed. I tried to make it as comprehensive and complete as I could.
You can refer to the MDN documentation for more information.
"use strict" a directive introduced in ECMAScript 5.
Directives are similar to statements, yet different.
use strict does not contain key words: The directive is a simple expression statement, which consists of a special string literal (in single or double quotes). JavaScript engines, that do not implement ECMAScript 5, merely see an expression statement without side effects. It is expected that future versions of ECMAScript standards introduce use as a real key word; the quotes would thereby become obsolete.
use strict can be used only at the beginning of a script or of a function, i.e. it must precede every other (real) statement. It does not have to be the first instruction in a script of function: it can be preceded by other statement expressions that consist of string literals ( and JavaScript implementations can treat them as implementation specific directives). String literals statements, which follow a first real statement (in a script or function) are simple expression statements. Interpreters must not interpret them as directives and they have no effect.
The use strict directive indicates that the following code (in a script or a function) is strict code.
The code in the highest level of a script (code that is not in a function) is considered strict code when the script contains a use strict directive.
The content of a function is considered strict code when the function itself is defined in a strict code or when the function contains a use strict directive.
Code that is passed to an eval() method is considered strict code when eval() was called from a strict code or contains the use strict directive itself.
The strict mode of ECMAScript 5 is a restricted subset of the JavaScript language, which eliminates relevant deficits of the language and features more stringent error checking and higher security. The following lists the differences between strict mode and normal mode (of which the first three are particularly important):
You cannot use the with-statement in strict mode.
In strict mode all variables have to be declared: if you assign a value to an identifier that has not been declared as variable, function, function parameter, catch-clause parameter or property of the global Object, then you will get a ReferenceError. In normal mode the identifier is implicitly declared as a global variable (as a property of the global Object)
In strict mode the keyword this has the value undefined in functions that were invoked as functions (not as methods). (In normal mode this always points to the global Object). This difference can be used to test if an implementation supports the strict mode:
var hasStrictMode = (function() { "use strict"; return this===undefined }());
Also when a function is invoked with call() or apply in strict mode, then this is exactly the value of the first argument of the call()or apply() invocation. (In normal mode null and undefined are replaced by the global Object and values, which are not objects, are cast into objects.)
In strict mode you will get a TypeError, when you try to assign to readonly properties or to define new properties for a non extensible object. (In normal mode both simply fail without error message.)
In strict mode, when passing code to eval(), you cannot declare or define variables or functions in the scope of the caller (as you can do it in normal mode). Instead, a new scope is created for eval() and the variables and functions are within that scope. That scope is destroyed after eval() finishes execution.
In strict mode the arguments-object of a function contains a static copy of the values, which are passed to that function. In normal mode the arguments-object has a somewhat "magical" behaviour: The elements of the array and the named function parameters reference both the same value.
In strict mode you will get a SyntaxError when the delete operator is followed by a non qualified identifier (a variable, function or function parameter). In normal mode the delete expression would do nothing and is evaluated to false.
In strict mode you will get a TypeError when you try to delete a non configurable property. (In normal mode the attempt simply fails and the delete expression is evaluated to false).
In strict mode it is considered a syntactical error when you try to define several properties with the same name for an object literal. (In normal mode there is no error.)
In strict mode it is considered a syntactical error when a function declaration has multiple parameters with the same name. (In normal mode there is no error.)
In strict mode octal literals are not allowed (these are literals that start with 0. (In normal mode some implementations do allow octal literals.)
In strict mode the identifiers eval and arguments are treated like keywords. You cannot change their value, cannot assign a value to them, and you cannot use them as names for variables, functions, function parameters or identifiers of a catch block.
In strict mode are more restrictions on the possibilities to examine the call stack. arguments.caller and arguments.callee cause a TypeError in a function in strict mode. Furthermore, some caller- and arguments properties of functions in strict mode cause a TypeError when you try to read them.

My two cents:
One of the goals of strict mode is to allow for faster debugging of issues. It helps the developers by throwing exception when certain wrong things occur that can cause silent & strange behaviour of your webpage. The moment we use use strict, the code will throw out errors which helps developer to fix it in advance.
Few important things which I have learned after using use strict :
Prevents Global Variable Declaration:
"use strict";
var tree1Data = { name: 'Banana Tree',age: 100,leafCount: 100000};
function Tree(typeOfTree) {
var age;
var leafCount;
age = typeOfTree.age;
leafCount = typeOfTree.leafCount;
nameoftree = typeOfTree.name;
};
var tree1 = new Tree(tree1Data);
console.log(window);
Now,this code creates nameoftree in global scope which could be accessed using window.nameoftree. When we implement use strict the code would throw error.
Uncaught ReferenceError: nameoftree is not defined
Eliminates with statement :
with statements can't be minified using tools like uglify-js. They're also deprecated and removed from future JavaScript versions.
Sample:
"use strict";
var tree1Data = {
name: 'Banana Tree',
age: 100,
leafCount: 100000
};
function Tree(typeOfTree) {
var age;
var leafCount;
age = typeOfTree.age;
leafCount = typeOfTree.leafCount;
// nameoftree = typeOfTree.name;
for (var i = 0; i < 2; ++i) {
// let(leafCount = i) { /*do something*/ }
}
for (var i = 0; i < 2; ++i) {
with(leafCount = i) { /*do something*/ }
}
};
var tree1 = new Tree(tree1Data);
console.log(window);
Prevents Duplicates :
When we have duplicate property, it throws an exception
Uncaught SyntaxError: Duplicate data property in object literal not
allowed in strict mode
"use strict";
var tree1Data = {
name: 'Banana Tree',
age: 100,
leafCount: 100000,
name:'Banana Tree'
};
There are few more but I need to gain more knowledge on that.

If you use a browser released in the last year or so then it most likely supports JavaScript Strict mode. Only older browsers around before ECMAScript 5 became the current standard don't support it.
The quotes around the command make sure that the code will still work in older browsers as well (although the things that generate a syntax error in strict mode will generally just cause the script to malfunction in some hard to detect way in those older browsers).

When adding "use strict";, the following cases will throw a SyntaxError before the script is executing:
Paving the way for future ECMAScript versions, using one of the newly reserved keywords (in prevision for ECMAScript 6): implements, interface, let, package, private, protected, public, static, and yield.
Declaring function in blocks
if(a<b){ function f(){} }
Octal syntax
var n = 023;
this point to the global object.
function f() {
"use strict";
this.a = 1;
};
f();
Declaring twice the same name for a property name in an object literal
{a: 1, b: 3, a: 7}
This is no longer the case in ECMAScript 6 (bug 1041128).
Declaring two function arguments with the same name function
f(a, b, b){}
Setting a value to an undeclared variable
function f(x){
"use strict";
var a = 12;
b = a + x*35; // error!
}
f();
Using delete on a variable name delete myVariable;
Using eval or arguments as variable or function argument name
"use strict";
arguments++;
var obj = { set p(arguments) { } };
try { } catch (arguments) { }
function arguments() { }
Sources:
Transitioning to strict mode on MDN
Strict mode on MDN
JavaScript’s Strict Mode and Why You Should Use It on Colin J. Ihrig's blog (archived version)

Strict mode makes several changes to normal JavaScript semantics:
eliminates some JavaScript silent errors by changing them
to throw errors.
fixes mistakes that make it difficult for JavaScript
engines to perform optimizations.
prohibits some syntax likely to be defined in future
versions of ECMAScript.
for more information vistit Strict Mode- Javascript

"Use Strict"; is an insurance that programmer will not use the loose or the bad properties of JavaScript. It is a guide, just like a ruler will help you make straight lines. "Use Strict" will help you do "Straight coding".
Those that prefer not to use rulers to do their lines straight usually end up in those pages asking for others to debug their code.
Believe me. The overhead is negligible compared to poorly designed code. Doug Crockford, who has been a senior JavaScript developer for several years, has a very interesting post here. Personally, I like to return to his site all the time to make sure I don't forget my good practice.
Modern JavaScript practice should always evoke the "Use Strict"; pragma. The only reason that the ECMA Group has made the "Strict" mode optional is to permit less experienced coders access to JavaScript and give then time to adapt to the new and safer coding practices.

Including use strict in the beginning of your all sensitive JavaScript files from this point is a small way to be a better JavaScript programmer and avoid random variables becoming global and things change silently.

Quoting from w3schools:
The "use strict" Directive
The "use strict" directive is new in JavaScript 1.8.5 (ECMAScript
version 5).
It is not a statement, but a literal expression, ignored by earlier
versions of JavaScript.
The purpose of "use strict" is to indicate that the code should be
executed in "strict mode".
With strict mode, you can not, for example, use undeclared variables.
Why Strict Mode?
Strict mode makes it easier to write "secure" JavaScript.
Strict mode changes previously accepted "bad syntax" into real errors.
As an example, in normal JavaScript, mistyping a variable name creates
a new global variable. In strict mode, this will throw an error,
making it impossible to accidentally create a global variable.
In normal JavaScript, a developer will not receive any error feedback
assigning values to non-writable properties.
In strict mode, any assignment to a non-writable property, a
getter-only property, a non-existing property, a non-existing
variable, or a non-existing object, will throw an error.
Please refer to http://www.w3schools.com/js/js_strict.asp to know more

"use strict" makes JavaScript code to run in strict mode, which basically means everything needs to be defined before use. The main reason for using strict mode is to avoid accidental global uses of undefined methods.
Also in strict mode, things run faster, some warnings or silent warnings throw fatal errors, it's better to always use it to make a neater code.
"use strict" is widely needed to be used in ECMA5, in ECMA6 it's part of JavaScript by default, so it doesn't need to be added if you're using ES6.
Look at these statements and examples from MDN:
The "use strict" Directive The "use strict" directive is new in
JavaScript 1.8.5 (ECMAScript version 5). It is not a statement, but a
literal expression, ignored by earlier versions of JavaScript. The
purpose of "use strict" is to indicate that the code should be
executed in "strict mode". With strict mode, you can not, for example,
use undeclared variables.
Examples of using "use strict":
Strict mode for functions: Likewise, to invoke strict mode for a
function, put the exact statement "use strict"; (or 'use strict';) in
the function's body before any other statements.
1) strict mode in functions
function strict() {
// Function-level strict mode syntax
'use strict';
function nested() { return 'And so am I!'; }
return "Hi! I'm a strict mode function! " + nested();
}
function notStrict() { return "I'm not strict."; }
console.log(strict(), notStrict());
2) whole-script strict mode
'use strict';
var v = "Hi! I'm a strict mode script!";
console.log(v);
3) Assignment to a non-writable global
'use strict';
// Assignment to a non-writable global
var undefined = 5; // throws a TypeError
var Infinity = 5; // throws a TypeError
// Assignment to a non-writable property
var obj1 = {};
Object.defineProperty(obj1, 'x', { value: 42, writable: false });
obj1.x = 9; // throws a TypeError
// Assignment to a getter-only property
var obj2 = { get x() { return 17; } };
obj2.x = 5; // throws a TypeError
// Assignment to a new property on a non-extensible object.
var fixed = {};
Object.preventExtensions(fixed);
fixed.newProp = 'ohai'; // throws a TypeError
You can read more on MDN.

There's a good talk by some people who were on the ECMAScript committee: Changes to JavaScript, Part 1: ECMAScript 5" about how incremental use of the "use strict" switch allows JavaScript implementers to clean up a lot of the dangerous features of JavaScript without suddenly breaking every website in the world.
Of course it also talks about just what a lot of those misfeatures are (were) and how ECMAScript 5 fixes them.

Small examples to compare:
Non-strict mode:
for (i of [1,2,3]) console.log(i)
// output:
// 1
// 2
// 3
Strict mode:
'use strict';
for (i of [1,2,3]) console.log(i)
// output:
// Uncaught ReferenceError: i is not defined
Non-strict mode:
String.prototype.test = function () {
console.log(typeof this === 'string');
};
'a'.test();
// output
// false
String.prototype.test = function () {
'use strict';
console.log(typeof this === 'string');
};
'a'.test();
// output
// true

Note that use strict was introduced in EcmaScript 5 and was kept since then.
Below are the conditions to trigger strict mode in ES6 and ES7:
Global code is strict mode code if it begins with a Directive Prologue that contains a Use Strict Directive (see 14.1.1).
Module code is always strict mode code.
All parts of a ClassDeclaration or a ClassExpression are strict mode code.
Eval code is strict mode code if it begins with a Directive Prologue that contains a Use Strict Directive or if the call to eval is a direct eval (see 12.3.4.1) that is contained in strict mode code.
Function code is strict mode code if the associated FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, MethodDefinition, or ArrowFunction is contained in strict mode code or if the code that produces the value of the function’s [[ECMAScriptCode]] internal slot begins with a Directive Prologue that contains a Use Strict Directive.
Function code that is supplied as the arguments to the built-in Function and Generator constructors is strict mode code if the last argument is a String that when processed is a FunctionBody that begins with a Directive Prologue that contains a Use Strict Directive.

The main reasons why developers should use "use strict" are:
Prevents accidental declaration of global variables.Using "use strict()" will make sure that variables are declared with var before use.
Eg:
function useStrictDemo(){
'use strict';
//works fine
var a = 'No Problem';
//does not work fine and throws error
k = "problem"
//even this will throw error
someObject = {'problem': 'lot of problem'};
}
N.B: The "use strict" directive is only recognized at the beginning of a script or a function.
The string "arguments" cannot be used as a variable:
"use strict";
var arguments = 3.14; // This will cause an error
Will restrict uses of keywords as variables. Trying to use them will throw errors.
In short will make your code less error prone and in turn will make you write good code.
To read more about it you can refer here.

use strict is a way to make your code safer, because you can't use dangerous features that can work not as you expect. And, as was written before, it makes code more strict.

JavaScript “strict” mode was introduced in ECMAScript 5.
(function() {
"use strict";
your code...
})();
Writing "use strict"; at the very top of your JS file turns on strict
syntax checking. It does the following tasks for us:
shows an error if you try to assign to an undeclared variable
stops you from overwriting key JS system libraries
forbids some unsafe or error-prone language features
use strict also works inside of individual functions. It is always a better practice to include use strict in your code.
Browser compatibility issue:
The "use" directives are meant to be backwards-compatible. Browsers that do not support them will just see a string literal that isn't referenced further. So, they will pass over it and move on.

"use strict"; is the ECMA effort to make JavaScript a little bit more robust. It brings in JS an attempt to make it at least a little "strict" (other languages implement strict rules since the 90s). It actually "forces" JavaScript developers to follow some sort of coding best practices.
Still, JavaScript is very fragile. There is no such thing as typed variables, typed methods, etc.
I strongly recommend JavaScript developers to learn a more robust language such as Java or ActionScript3, and implement the same best practices in your JavaScript code, it will work better and be easier to debug.

Normally, JavaScript does not follow strict rules, hence increasing chances of errors. After using "use strict", the JavaScript code should follow strict set of rules as in other programming languages such as use of terminators, declaration before initialization, etc.
If "use strict" is used, the code should be written by following a strict set of rules, hence decreasing the chances of errors and ambiguities.

Use Strict is used to show common and repeated errors so that it is handled differently , and changes the way java script runs , such changes are :
Prevents accidental globals
No duplicates
Eliminates with
Eliminates this coercion
Safer eval()
Errors for immutables
you can also read this article for the details

"use strict"; Defines that JavaScript code should be executed in
"strict mode".
The "use strict" directive was new in ECMAScript version 5.
It is not a statement, but a literal expression, ignored by earlier
versions of JavaScript.
The purpose of "use strict" is to indicate that the code should be
executed in "strict mode".
With strict mode, you can not, for example, use undeclared variables.
All modern browsers support "use strict" except Internet Explorer 9 and lower.
Disadvantage
If a developer used a library that was in strict mode, but the developer was used to working in normal mode, they might call some actions on the library that wouldn’t work as expected.
Worse, since the developer is in normal mode, they don’t have the advantages of extra errors being thrown, so the error might fail silently.
Also, as listed above, strict mode stops you from doing certain things.
People generally think that you shouldn’t use those things in the first place, but some developers don’t like the constraint and want to use all the features of the language.
For basic example and for reference go through :
https://www.tutorialsteacher.com/javascript/javascript-strict

JavaScript was designed and implemented hastily because of the browser wars and bad management. As a result many poor design decisions, un-intuitive syntax and confusing semantics found their way into the language. Strict mode aims to amend some of these mistakes.
But fixing these mistakes without creating alternative interpretation breaks backward compatibility. So, "use strict" directive creates that alternative interpretation of the code while communicating it to the programmer.
For example, this keywords refers to the object in a method definition, like this or self in other languages.
let o = {
name: 'John Doe',
sayName: function(){
console.log(this.name);
}
};
o.sayName(); // 'John Doe'
this has no purpose outside the method context but all JavaScript functions have this keyword whether they are methods or not:
function run() {
console.log(this);
}
run(); // Window
Here this resolves to the global object which does not make sense and serves no purpose because global object is already available in the scope.
In strict mode this in a global function resolves to undefined, which is what we expect.
"use strict"
function run() {
console.log(this);
}
run(); // undefined
Some mistakes can not be fixed even in strict mode because syntax should be valid for older browsers since they ignore "strict mode" directive. This is by design.

Strict mode can prevent memory leaks.
Please check the function below written in non-strict mode:
function getname(){
name = "Stack Overflow"; // Not using var keyword
return name;
}
getname();
console.log(name); // Stack Overflow
In this function, we are using a variable called name inside the function. Internally, the compiler will first check if there is any variable declared with that particular name in that particular function scope. Since the compiler understood that there is no such variable, it will check in the outer scope. In our case, it is the global scope. Again, the compiler understood that there is also no variable declared in the global space with that name, so it creates such a variable for us in the global space. Conceptually, this variable will be created in the global scope and will be available in the entire application.
Another scenario is that, say, the variable is declared in a child function. In that case, the compiler checks the validity of that variable in the outer scope, i.e., the parent function. Only then it will check in the global space and create a variable for us there.
That means additional checks need to be done. This will affect the performance of the application.
Now let's write the same function in strict mode.
"use strict"
function getname(){
name = "Stack Overflow"; // Not using var keyword
return name;
}
getname();
console.log(name);
We will get the following error.
Uncaught ReferenceError: name is not defined
at getname (<anonymous>:3:15)
at <anonymous>:6:5
Here, the compiler throws the reference error. In strict mode, the compiler does not allow us to use the variable without declaring it. So memory leaks can be prevented. In addition, we can write more optimized code.

Strict mode eliminates errors that would be ignored in non-strict mode, thus making javascript “more secured”.
Is it considered among best practices?
Yes, It's considered part of the best practices while working with javascript to include Strict mode. This is done by adding the below line of code in your JS file.
'use strict';
in your code.
What does it mean to user agents?
Indicating that code should be interpreted in strict mode specifies to user agents like browsers that they should treat code literally as written, and throw an error if the code doesn't make sense.
For example: Consider in your .js file you have the following code:
Scenario 1: [NO STRICT MODE]
var city = "Chicago"
console.log(city) // Prints the city name, i.e. Chicago
Scenario 2: [NO STRICT MODE]
city = "Chicago"
console.log(city) // Prints the city name, i.e. Chicago
So why does the variable name is being printed in both cases?
Without strict mode turned on, user agents often go through a series of modifications to problematic code in an attempt to get it to make sense. On the surface, this can seem like a fine thing, and indeed, working outside of strict mode makes it possible for people to get their feet wet with JavaScript code without having all the details quite nailed down. However, as a developer, I don't want to leave a bug in my code, because I know it could come back and bite me later on, and I also just want to write good code. And that's where strict mode helps out.
Scenario 3: [STRICT MODE]
'use strict';
city = "Chicago"
console.log(city) // Reference Error: asignment is undeclared variable city.
Additional tip: To maintain code quality using strict mode, you don't need to write this over and again especially if you have multiple .js file. You can enforce this rule globally in eslint rules as follows:
Filename: .eslintrc.js
module.exports = {
env: {
es6: true
},
rules : {
strict: ['error', 'global'],
},
};
Okay, so what is prevented in strict mode?
Using a variable without declaring it will throw an error in strict mode. This is to prevent unintentionally creating global variables throughout your application. The example with printing Chicago covers this in particular.
Deleting a variable or a function or an argument is a no-no in strict mode.
"use strict";
function x(p1, p2) {};
delete x; // This will cause an error
Duplicating a parameter name is not allowed in strict mode.
"use strict";
function x(p1, p1) {}; // This will cause an error
Reserved words in the Javascript language are not allowed in strict mode. The words are implements interface, let, packages, private, protected, public. static, and yield
For a more comprehensive list check out the MDN documentation here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode

strict mode enables strict features in the v8 engine. Short example of some features:
You can enable it globally by writing:
'use strict'; // strict mode enabled!
Per function you just include in function:
let myfunc = () => {
'use strict'; // strict mode enabled
b = 0; // broke
}
You MUST declare a variable before using it (sane imo):
var x;
x = '0'; // ok
y = ''; // not ok
es6 features are enabled (this is browser dependent), for node v4+ this is important.
Performance, in some cases, is better.
There are more features as well, check here for more!

What does JavaScript's strict mode exactly do regarding implicit global declarations?

From MDN article about strict mode:
First, strict mode makes it impossible to accidentally create global
variables. In normal JavaScript mistyping a variable in an assignment
creates a new property on the global object and continues to "work"
(although future failure is possible: likely, in modern JavaScript).
Assignments, which would accidentally create global variables, instead
throw an error in strict mode:
'use strict';
// Assuming a global variable mistypedVariable exists
mistypeVariable = 17; // this line throws a ReferenceError due to the
// misspelling of variable
What does this mean? Does the engine detect if a variable with a similar name already exists(?!) or does strict mode merely prohibit declaring global variables outside of global scope?
The above quotation seems to hint the first possibility but this seems... weird?

The comment in the quoted code is misleading: The "Assuming" part is irrelevant. It should really read: "Assuming that no global variable with exactly this name was defined with let or var,..."
But do note what the purpose was of the code snippet and the comment: to show that in strict mode it is much easier to spot spelling mistakes. In sloppy mode the spelling mistake would go unnoticed (no errors) and you would be working with two variables without knowing it (at least for a while)

As requested, I'll make this into an answer :)
I think you understand what happens here already, and are just getting confused through reading that quote about "assuming a global variable..." too literally. (I will admit it is phrased in such a way as to possibly lead to this confusion though.) The real situation is very simple, and has nothing to do with variables having "similar names" (which JS has no concept of):
What is being talked about is what happens if you assign a value to a variable which has not been formally declared (variables are declared with one of the keywords var, let or const). Not declaring your variables is bad practice, and in strict mode will throw an error - this is a good thing and warns you of your mistake. But in non-strict mode, JS will happily accept this and think you wanted to declare a global variable of that name. This is almost never what you actually want, as it pollutes the global namespace, doesn't inform you of your error, and may lead to all sorts of tricky bugs later.

does strict mode merely prohibit declaring global variables outside of global scope?
strict mode definitely doesn't prohibit declaring global variables from anywhere. In non-strict mode, if you write:
someVar = 't';
it will evaluate to:
window.someVar = 't';
( why is this happening? ) despite of writing inside or outside of a function scope. actually, the line was both declaring and evaluation of variable ( look again, it doesn't have var so it shouldn't declare anything! ).
But it would cause such a side-effect that wasn't totally fine and they've introduced strict-mode which when is active, our first line will throw an error. because it's just evaluation without declaring it first.
now if you need to manipulate global scope inside of a function scope, you only should global object as reference:
var someGlobalVar;
var someOtherGlobalVar;
function hello() {
// this will make *someGlobalVar* to be redefined in the inner scope
// someGlobalVar != window.someGlobalVar
var someGlobalVar;
// manipulating inner variable, not global one
someGlobalVar = 's';
// it's fine, you've accessed global object correctly
window.someGlobalVar = 's';
// you're still editing global object, because
// there's no other variable inside of this scope having the same name
someOtherGlobalVar = 's';
}

Is 'use strict' a must to use? [duplicate]

Recently, I ran some of my JavaScript code through Crockford's JSLint, and it gave the following error:
Problem at line 1 character 1: Missing "use strict" statement.
Doing some searching, I realized that some people add "use strict"; into their JavaScript code. Once I added the statement, the error stopped appearing. Unfortunately, Google did not reveal much of the history behind this string statement. Certainly it must have something to do with how the JavaScript is interpreted by the browser, but I have no idea what the effect would be.
So what is "use strict"; all about, what does it imply, and is it still relevant?
Do any of the current browsers respond to the "use strict"; string or is it for future use?

Update for ES6 modules
Inside native ECMAScript modules (with import and export statements) and ES6 classes, strict mode is always enabled and cannot be disabled.
Original answer
This article about Javascript Strict Mode might interest you: John Resig - ECMAScript 5 Strict Mode, JSON, and More
To quote some interesting parts:
Strict Mode is a new feature in ECMAScript 5 that allows you to place a program, or a function, in a "strict" operating context. This strict context prevents certain actions from being taken and throws more exceptions.
And:
Strict mode helps out in a couple ways:
It catches some common coding bloopers, throwing exceptions.
It prevents, or throws errors, when relatively "unsafe" actions are taken (such as gaining access to the global object).
It disables features that are confusing or poorly thought out.
Also note you can apply "strict mode" to the whole file... Or you can use it only for a specific function (still quoting from John Resig's article):
// Non-strict code...
(function(){
"use strict";
// Define your library strictly...
})();
// Non-strict code...
Which might be helpful if you have to mix old and new code ;-)
So, I suppose it's a bit like the "use strict" you can use in Perl (hence the name?): it helps you make fewer errors, by detecting more things that could lead to breakages.
Strict mode is now supported by all major browsers.

It's a new feature of ECMAScript 5. John Resig wrote up a nice summary of it.
It's just a string you put in your JavaScript files (either at the top of your file or inside of a function) that looks like this:
"use strict";
Putting it in your code now shouldn't cause any problems with current browsers as it's just a string. It may cause problems with your code in the future if your code violates the pragma. For instance, if you currently have foo = "bar" without defining foo first, your code will start failing...which is a good thing in my opinion.

The statement "use strict"; instructs the browser to use the Strict mode, which is a reduced and safer feature set of JavaScript.
List of features (non-exhaustive)
Disallows global variables. (Catches missing var declarations and typos in variable names)
Silent failing assignments will throw error in strict mode (assigning NaN = 5;)
Attempts to delete undeletable properties will throw (delete Object.prototype)
Requires all property names in an object literal to be unique (var x = {x1: "1", x1: "2"})
Function parameter names must be unique (function sum (x, x) {...})
Forbids octal syntax (var x = 023; some devs assume wrongly that a preceding zero does nothing to change the number.)
Forbids the with keyword
eval in strict mode does not introduce new variables
Forbids deleting plain names (delete x;)
Forbids binding or assignment of the names eval and arguments in any form
Strict mode does not alias properties of the arguments object with the formal parameters. (e.g. in function sum (a,b) { return arguments[0] + b;} This works because arguments[0] is bound to a and so on. ) (See examples section below to understand the difference)
arguments.callee is not supported
[Ref: Strict mode, Mozilla Developer Network]
Examples:
Strict mode code doesn't alias properties of arguments objects created within it
function show( msg ){
msg = 42;
console.log( msg ); // msg === 42
console.log( arguments[0] ); // arguments === 42
}
show( "Hey" );
// In strict mode arguments[i] does not track the value of
// the corresponding named argument, nor does a named argument track the value in the corresponding arguments[i]
function showStrict( msg ){
"use strict";
msg = 42;
console.log( msg ); // msg === 42
console.log( arguments[0] ); // arguments === "Hey"
}
showStrict( "Hey" );

If people are worried about using use strict it might be worth checking out this article:
ECMAScript 5 'Strict mode' support in browsers. What does this mean?
NovoGeek.com - Krishna's weblog
It talks about browser support, but more importantly how to deal with it safely:
function isStrictMode(){
return !this;
}
/*
returns false, since 'this' refers to global object and
'!this' becomes false
*/
function isStrictMode(){
"use strict";
return !this;
}
/*
returns true, since in strict mode the keyword 'this'
does not refer to global object, unlike traditional JS.
So here, 'this' is 'undefined' and '!this' becomes true.
*/

A word of caution, all you hard-charging programmers: applying "use strict" to existing code can be hazardous! This thing is not some feel-good, happy-face sticker that you can slap on the code to make it 'better'. With the "use strict" pragma, the browser will suddenly THROW exceptions in random places that it never threw before just because at that spot you are doing something that default/loose JavaScript happily allows but strict JavaScript abhors! You may have strictness violations hiding in seldom used calls in your code that will only throw an exception when they do eventually get run - say, in the production environment that your paying customers use!
If you are going to take the plunge, it is a good idea to apply "use strict" alongside comprehensive unit tests and a strictly configured JSHint build task that will give you some confidence that there is no dark corner of your module that will blow up horribly just because you've turned on Strict Mode. Or, hey, here's another option: just don't add "use strict" to any of your legacy code, it's probably safer that way, honestly. DEFINITELY DO NOT add "use strict" to any modules you do not own or maintain, like third party modules.
I think even though it is a deadly caged animal, "use strict" can be good stuff, but you have to do it right. The best time to go strict is when your project is greenfield and you are starting from scratch. Configure JSHint/JSLint with all the warnings and options cranked up as tight as your team can stomach, get a good build/test/assert system du jour rigged like Grunt+Karma+Chai, and only THEN start marking all your new modules as "use strict". Be prepared to cure lots of niggly errors and warnings. Make sure everyone understands the gravity by configuring the build to FAIL if JSHint/JSLint produces any violations.
My project was not a greenfield project when I adopted "use strict". As a result, my IDE is full of red marks because I don't have "use strict" on half my modules, and JSHint complains about that. It's a reminder to me about what refactoring I should do in the future. My goal is to be red mark free due to all of my missing "use strict" statements, but that is years away now.

Using 'use strict'; does not suddenly make your code better.
The JavaScript strict mode is a feature in ECMAScript 5. You can enable the strict mode by declaring this in the top of your script/function.
'use strict';
When a JavaScript engine sees this directive, it will start to interpret the code in a special mode. In this mode, errors are thrown up when certain coding practices that could end up being potential bugs are detected (which is the reasoning behind the strict mode).
Consider this example:
var a = 365;
var b = 030;
In their obsession to line up the numeric literals, the developer has inadvertently initialized variable b with an octal literal. Non-strict mode will interpret this as a numeric literal with value 24 (in base 10). However, strict mode will throw an error.
For a non-exhaustive list of specialties in strict mode, see this answer.
Where should I use 'use strict';?
In my new JavaScript application: Absolutely! Strict mode can be used as a whistleblower when you are doing something stupid with your code.
In my existing JavaScript code: Probably not! If your existing JavaScript code has statements that are prohibited in strict-mode, the application will simply break. If you want strict mode, you should be prepared to debug and correct your existing code. This is why using 'use strict'; does not suddenly make your code better.
How do I use strict mode?
Insert a 'use strict'; statement on top of your script:
// File: myscript.js
'use strict';
var a = 2;
....
Note that everything in the file myscript.js will be interpreted in strict mode.
Or, insert a 'use strict'; statement on top of your function body:
function doSomething() {
'use strict';
...
}
Everything in the lexical scope of function doSomething will be interpreted in strict mode. The word lexical scope is important here. For example, if your strict code calls a function of a library that is not strict, only your code is executed in strict mode, and not the called function. See this answer for a better explanation.
What things are prohibited in strict mode?
I found a nice article describing several things that are prohibited in strict mode (note that this is not an exhaustive list):
Scope
Historically, JavaScript has been confused about how functions
are scoped. Sometimes they seem to be statically scoped, but some
features make them behave like they are dynamically scoped. This is
confusing, making programs difficult to read and understand.
Misunderstanding causes bugs. It also is a problem for performance.
Static scoping would permit variable binding to happen at compile
time, but the requirement for dynamic scope means the binding must be
deferred to runtime, which comes with a significant performance
penalty.
Strict mode requires that all variable binding be done statically.
That means that the features that previously required dynamic binding
must be eliminated or modified. Specifically, the with statement is
eliminated, and the eval function’s ability to tamper with the
environment of its caller is severely restricted.
One of the benefits of strict code is that tools like YUI Compressor
can do a better job when processing it.
Implied Global Variables
JavaScript has implied global variables. If
you do not explicitly declare a variable, a global variable is
implicitly declared for you. This makes programming easier for
beginners because they can neglect some of their basic housekeeping
chores. But it makes the management of larger programs much more
difficult and it significantly degrades reliability. So in strict
mode, implied global variables are no longer created. You should
explicitly declare all of your variables.
Global Leakage
There are a number of situations that could cause this
to be bound to the global object. For example, if you forget to
provide the new prefix when calling a constructor function, the
constructor's this will be bound unexpectedly to the global object, so
instead of initializing a new object, it will instead be silently
tampering with global variables. In these situations, strict mode will
instead bind this to undefined, which will cause the constructor to
throw an exception instead, allowing the error to be detected much
sooner.
Noisy Failure
JavaScript has always had read-only properties, but you
could not create them yourself until ES5’s Object.createProperty
function exposed that capability. If you attempted to assign a value
to a read-only property, it would fail silently. The assignment would
not change the property’s value, but your program would proceed as
though it had. This is an integrity hazard that can cause programs to
go into an inconsistent state. In strict mode, attempting to change a
read-only property will throw an exception.
Octal
The octal (or base 8) representation of numbers was extremely
useful when doing machine-level programming on machines whose word
sizes were a multiple of 3. You needed octal when working with the CDC
6600 mainframe, which had a word size of 60 bits. If you could read
octal, you could look at a word as 20 digits. Two digits represented
the op code, and one digit identified one of 8 registers. During the
slow transition from machine codes to high level languages, it was
thought to be useful to provide octal forms in programming languages.
In C, an extremely unfortunate representation of octalness was
selected: Leading zero. So in C, 0100 means 64, not 100, and 08 is an
error, not 8. Even more unfortunately, this anachronism has been
copied into nearly all modern languages, including JavaScript, where
it is only used to create errors. It has no other purpose. So in
strict mode, octal forms are no longer allowed.
Et cetera
The arguments pseudo array becomes a little bit more
array-like in ES5. In strict mode, it loses its callee and caller
properties. This makes it possible to pass your arguments to untrusted
code without giving up a lot of confidential context. Also, the
arguments property of functions is eliminated.
In strict mode, duplicate keys in a function literal will produce a
syntax error. A function can’t have two parameters with the same name.
A function can’t have a variable with the same name as one of its
parameters. A function can’t delete its own variables. An attempt to
delete a non-configurable property now throws an exception. Primitive
values are not implicitly wrapped.
Reserved words for future JavaScript versions
ECMAScript 5 adds a list of reserved words. If you use them as variables or arguments, strict mode will throw an error. The reserved words are:
implements, interface, let, package, private, protected, public, static, and yield
Further Reading
Strict Mode - JavaScript | MDN
Browser support for strict mode
Transitioning to strict mode

I strongly recommend every developer to start using strict mode now. There are enough browsers supporting it that strict mode will legitimately help save us from errors we didn’t even know were in your code.
Apparently, at the initial stage there will be errors we have never encountered before. To get the full benefit, we need to do proper testing after switching to strict mode to make sure we have caught everything. Definitely we don’t just throw use strict in our code and assume there are no errors. So the churn is that it’s time to start using this incredibly useful language feature to write better code.
For example,
var person = {
name : 'xyz',
position : 'abc',
fullname : function () { "use strict"; return this.name; }
};
JSLint is a debugger written by Douglas Crockford. Simply paste in your script, and it’ll quickly scan for any noticeable issues and errors in your code.

I would like to offer a somewhat more founded answer complementing the other answers. I was hoping to edit the most popular answer, but failed. I tried to make it as comprehensive and complete as I could.
You can refer to the MDN documentation for more information.
"use strict" a directive introduced in ECMAScript 5.
Directives are similar to statements, yet different.
use strict does not contain key words: The directive is a simple expression statement, which consists of a special string literal (in single or double quotes). JavaScript engines, that do not implement ECMAScript 5, merely see an expression statement without side effects. It is expected that future versions of ECMAScript standards introduce use as a real key word; the quotes would thereby become obsolete.
use strict can be used only at the beginning of a script or of a function, i.e. it must precede every other (real) statement. It does not have to be the first instruction in a script of function: it can be preceded by other statement expressions that consist of string literals ( and JavaScript implementations can treat them as implementation specific directives). String literals statements, which follow a first real statement (in a script or function) are simple expression statements. Interpreters must not interpret them as directives and they have no effect.
The use strict directive indicates that the following code (in a script or a function) is strict code.
The code in the highest level of a script (code that is not in a function) is considered strict code when the script contains a use strict directive.
The content of a function is considered strict code when the function itself is defined in a strict code or when the function contains a use strict directive.
Code that is passed to an eval() method is considered strict code when eval() was called from a strict code or contains the use strict directive itself.
The strict mode of ECMAScript 5 is a restricted subset of the JavaScript language, which eliminates relevant deficits of the language and features more stringent error checking and higher security. The following lists the differences between strict mode and normal mode (of which the first three are particularly important):
You cannot use the with-statement in strict mode.
In strict mode all variables have to be declared: if you assign a value to an identifier that has not been declared as variable, function, function parameter, catch-clause parameter or property of the global Object, then you will get a ReferenceError. In normal mode the identifier is implicitly declared as a global variable (as a property of the global Object)
In strict mode the keyword this has the value undefined in functions that were invoked as functions (not as methods). (In normal mode this always points to the global Object). This difference can be used to test if an implementation supports the strict mode:
var hasStrictMode = (function() { "use strict"; return this===undefined }());
Also when a function is invoked with call() or apply in strict mode, then this is exactly the value of the first argument of the call()or apply() invocation. (In normal mode null and undefined are replaced by the global Object and values, which are not objects, are cast into objects.)
In strict mode you will get a TypeError, when you try to assign to readonly properties or to define new properties for a non extensible object. (In normal mode both simply fail without error message.)
In strict mode, when passing code to eval(), you cannot declare or define variables or functions in the scope of the caller (as you can do it in normal mode). Instead, a new scope is created for eval() and the variables and functions are within that scope. That scope is destroyed after eval() finishes execution.
In strict mode the arguments-object of a function contains a static copy of the values, which are passed to that function. In normal mode the arguments-object has a somewhat "magical" behaviour: The elements of the array and the named function parameters reference both the same value.
In strict mode you will get a SyntaxError when the delete operator is followed by a non qualified identifier (a variable, function or function parameter). In normal mode the delete expression would do nothing and is evaluated to false.
In strict mode you will get a TypeError when you try to delete a non configurable property. (In normal mode the attempt simply fails and the delete expression is evaluated to false).
In strict mode it is considered a syntactical error when you try to define several properties with the same name for an object literal. (In normal mode there is no error.)
In strict mode it is considered a syntactical error when a function declaration has multiple parameters with the same name. (In normal mode there is no error.)
In strict mode octal literals are not allowed (these are literals that start with 0. (In normal mode some implementations do allow octal literals.)
In strict mode the identifiers eval and arguments are treated like keywords. You cannot change their value, cannot assign a value to them, and you cannot use them as names for variables, functions, function parameters or identifiers of a catch block.
In strict mode are more restrictions on the possibilities to examine the call stack. arguments.caller and arguments.callee cause a TypeError in a function in strict mode. Furthermore, some caller- and arguments properties of functions in strict mode cause a TypeError when you try to read them.

My two cents:
One of the goals of strict mode is to allow for faster debugging of issues. It helps the developers by throwing exception when certain wrong things occur that can cause silent & strange behaviour of your webpage. The moment we use use strict, the code will throw out errors which helps developer to fix it in advance.
Few important things which I have learned after using use strict :
Prevents Global Variable Declaration:
"use strict";
var tree1Data = { name: 'Banana Tree',age: 100,leafCount: 100000};
function Tree(typeOfTree) {
var age;
var leafCount;
age = typeOfTree.age;
leafCount = typeOfTree.leafCount;
nameoftree = typeOfTree.name;
};
var tree1 = new Tree(tree1Data);
console.log(window);
Now,this code creates nameoftree in global scope which could be accessed using window.nameoftree. When we implement use strict the code would throw error.
Uncaught ReferenceError: nameoftree is not defined
Eliminates with statement :
with statements can't be minified using tools like uglify-js. They're also deprecated and removed from future JavaScript versions.
Sample:
"use strict";
var tree1Data = {
name: 'Banana Tree',
age: 100,
leafCount: 100000
};
function Tree(typeOfTree) {
var age;
var leafCount;
age = typeOfTree.age;
leafCount = typeOfTree.leafCount;
// nameoftree = typeOfTree.name;
for (var i = 0; i < 2; ++i) {
// let(leafCount = i) { /*do something*/ }
}
for (var i = 0; i < 2; ++i) {
with(leafCount = i) { /*do something*/ }
}
};
var tree1 = new Tree(tree1Data);
console.log(window);
Prevents Duplicates :
When we have duplicate property, it throws an exception
Uncaught SyntaxError: Duplicate data property in object literal not
allowed in strict mode
"use strict";
var tree1Data = {
name: 'Banana Tree',
age: 100,
leafCount: 100000,
name:'Banana Tree'
};
There are few more but I need to gain more knowledge on that.

If you use a browser released in the last year or so then it most likely supports JavaScript Strict mode. Only older browsers around before ECMAScript 5 became the current standard don't support it.
The quotes around the command make sure that the code will still work in older browsers as well (although the things that generate a syntax error in strict mode will generally just cause the script to malfunction in some hard to detect way in those older browsers).

When adding "use strict";, the following cases will throw a SyntaxError before the script is executing:
Paving the way for future ECMAScript versions, using one of the newly reserved keywords (in prevision for ECMAScript 6): implements, interface, let, package, private, protected, public, static, and yield.
Declaring function in blocks
if(a<b){ function f(){} }
Octal syntax
var n = 023;
this point to the global object.
function f() {
"use strict";
this.a = 1;
};
f();
Declaring twice the same name for a property name in an object literal
{a: 1, b: 3, a: 7}
This is no longer the case in ECMAScript 6 (bug 1041128).
Declaring two function arguments with the same name function
f(a, b, b){}
Setting a value to an undeclared variable
function f(x){
"use strict";
var a = 12;
b = a + x*35; // error!
}
f();
Using delete on a variable name delete myVariable;
Using eval or arguments as variable or function argument name
"use strict";
arguments++;
var obj = { set p(arguments) { } };
try { } catch (arguments) { }
function arguments() { }
Sources:
Transitioning to strict mode on MDN
Strict mode on MDN
JavaScript’s Strict Mode and Why You Should Use It on Colin J. Ihrig's blog (archived version)

Strict mode makes several changes to normal JavaScript semantics:
eliminates some JavaScript silent errors by changing them
to throw errors.
fixes mistakes that make it difficult for JavaScript
engines to perform optimizations.
prohibits some syntax likely to be defined in future
versions of ECMAScript.
for more information vistit Strict Mode- Javascript

"Use Strict"; is an insurance that programmer will not use the loose or the bad properties of JavaScript. It is a guide, just like a ruler will help you make straight lines. "Use Strict" will help you do "Straight coding".
Those that prefer not to use rulers to do their lines straight usually end up in those pages asking for others to debug their code.
Believe me. The overhead is negligible compared to poorly designed code. Doug Crockford, who has been a senior JavaScript developer for several years, has a very interesting post here. Personally, I like to return to his site all the time to make sure I don't forget my good practice.
Modern JavaScript practice should always evoke the "Use Strict"; pragma. The only reason that the ECMA Group has made the "Strict" mode optional is to permit less experienced coders access to JavaScript and give then time to adapt to the new and safer coding practices.

Including use strict in the beginning of your all sensitive JavaScript files from this point is a small way to be a better JavaScript programmer and avoid random variables becoming global and things change silently.

Quoting from w3schools:
The "use strict" Directive
The "use strict" directive is new in JavaScript 1.8.5 (ECMAScript
version 5).
It is not a statement, but a literal expression, ignored by earlier
versions of JavaScript.
The purpose of "use strict" is to indicate that the code should be
executed in "strict mode".
With strict mode, you can not, for example, use undeclared variables.
Why Strict Mode?
Strict mode makes it easier to write "secure" JavaScript.
Strict mode changes previously accepted "bad syntax" into real errors.
As an example, in normal JavaScript, mistyping a variable name creates
a new global variable. In strict mode, this will throw an error,
making it impossible to accidentally create a global variable.
In normal JavaScript, a developer will not receive any error feedback
assigning values to non-writable properties.
In strict mode, any assignment to a non-writable property, a
getter-only property, a non-existing property, a non-existing
variable, or a non-existing object, will throw an error.
Please refer to http://www.w3schools.com/js/js_strict.asp to know more

"use strict" makes JavaScript code to run in strict mode, which basically means everything needs to be defined before use. The main reason for using strict mode is to avoid accidental global uses of undefined methods.
Also in strict mode, things run faster, some warnings or silent warnings throw fatal errors, it's better to always use it to make a neater code.
"use strict" is widely needed to be used in ECMA5, in ECMA6 it's part of JavaScript by default, so it doesn't need to be added if you're using ES6.
Look at these statements and examples from MDN:
The "use strict" Directive The "use strict" directive is new in
JavaScript 1.8.5 (ECMAScript version 5). It is not a statement, but a
literal expression, ignored by earlier versions of JavaScript. The
purpose of "use strict" is to indicate that the code should be
executed in "strict mode". With strict mode, you can not, for example,
use undeclared variables.
Examples of using "use strict":
Strict mode for functions: Likewise, to invoke strict mode for a
function, put the exact statement "use strict"; (or 'use strict';) in
the function's body before any other statements.
1) strict mode in functions
function strict() {
// Function-level strict mode syntax
'use strict';
function nested() { return 'And so am I!'; }
return "Hi! I'm a strict mode function! " + nested();
}
function notStrict() { return "I'm not strict."; }
console.log(strict(), notStrict());
2) whole-script strict mode
'use strict';
var v = "Hi! I'm a strict mode script!";
console.log(v);
3) Assignment to a non-writable global
'use strict';
// Assignment to a non-writable global
var undefined = 5; // throws a TypeError
var Infinity = 5; // throws a TypeError
// Assignment to a non-writable property
var obj1 = {};
Object.defineProperty(obj1, 'x', { value: 42, writable: false });
obj1.x = 9; // throws a TypeError
// Assignment to a getter-only property
var obj2 = { get x() { return 17; } };
obj2.x = 5; // throws a TypeError
// Assignment to a new property on a non-extensible object.
var fixed = {};
Object.preventExtensions(fixed);
fixed.newProp = 'ohai'; // throws a TypeError
You can read more on MDN.

There's a good talk by some people who were on the ECMAScript committee: Changes to JavaScript, Part 1: ECMAScript 5" about how incremental use of the "use strict" switch allows JavaScript implementers to clean up a lot of the dangerous features of JavaScript without suddenly breaking every website in the world.
Of course it also talks about just what a lot of those misfeatures are (were) and how ECMAScript 5 fixes them.

Small examples to compare:
Non-strict mode:
for (i of [1,2,3]) console.log(i)
// output:
// 1
// 2
// 3
Strict mode:
'use strict';
for (i of [1,2,3]) console.log(i)
// output:
// Uncaught ReferenceError: i is not defined
Non-strict mode:
String.prototype.test = function () {
console.log(typeof this === 'string');
};
'a'.test();
// output
// false
String.prototype.test = function () {
'use strict';
console.log(typeof this === 'string');
};
'a'.test();
// output
// true

Note that use strict was introduced in EcmaScript 5 and was kept since then.
Below are the conditions to trigger strict mode in ES6 and ES7:
Global code is strict mode code if it begins with a Directive Prologue that contains a Use Strict Directive (see 14.1.1).
Module code is always strict mode code.
All parts of a ClassDeclaration or a ClassExpression are strict mode code.
Eval code is strict mode code if it begins with a Directive Prologue that contains a Use Strict Directive or if the call to eval is a direct eval (see 12.3.4.1) that is contained in strict mode code.
Function code is strict mode code if the associated FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, MethodDefinition, or ArrowFunction is contained in strict mode code or if the code that produces the value of the function’s [[ECMAScriptCode]] internal slot begins with a Directive Prologue that contains a Use Strict Directive.
Function code that is supplied as the arguments to the built-in Function and Generator constructors is strict mode code if the last argument is a String that when processed is a FunctionBody that begins with a Directive Prologue that contains a Use Strict Directive.

The main reasons why developers should use "use strict" are:
Prevents accidental declaration of global variables.Using "use strict()" will make sure that variables are declared with var before use.
Eg:
function useStrictDemo(){
'use strict';
//works fine
var a = 'No Problem';
//does not work fine and throws error
k = "problem"
//even this will throw error
someObject = {'problem': 'lot of problem'};
}
N.B: The "use strict" directive is only recognized at the beginning of a script or a function.
The string "arguments" cannot be used as a variable:
"use strict";
var arguments = 3.14; // This will cause an error
Will restrict uses of keywords as variables. Trying to use them will throw errors.
In short will make your code less error prone and in turn will make you write good code.
To read more about it you can refer here.

use strict is a way to make your code safer, because you can't use dangerous features that can work not as you expect. And, as was written before, it makes code more strict.

JavaScript “strict” mode was introduced in ECMAScript 5.
(function() {
"use strict";
your code...
})();
Writing "use strict"; at the very top of your JS file turns on strict
syntax checking. It does the following tasks for us:
shows an error if you try to assign to an undeclared variable
stops you from overwriting key JS system libraries
forbids some unsafe or error-prone language features
use strict also works inside of individual functions. It is always a better practice to include use strict in your code.
Browser compatibility issue:
The "use" directives are meant to be backwards-compatible. Browsers that do not support them will just see a string literal that isn't referenced further. So, they will pass over it and move on.

"use strict"; is the ECMA effort to make JavaScript a little bit more robust. It brings in JS an attempt to make it at least a little "strict" (other languages implement strict rules since the 90s). It actually "forces" JavaScript developers to follow some sort of coding best practices.
Still, JavaScript is very fragile. There is no such thing as typed variables, typed methods, etc.
I strongly recommend JavaScript developers to learn a more robust language such as Java or ActionScript3, and implement the same best practices in your JavaScript code, it will work better and be easier to debug.

Normally, JavaScript does not follow strict rules, hence increasing chances of errors. After using "use strict", the JavaScript code should follow strict set of rules as in other programming languages such as use of terminators, declaration before initialization, etc.
If "use strict" is used, the code should be written by following a strict set of rules, hence decreasing the chances of errors and ambiguities.

Use Strict is used to show common and repeated errors so that it is handled differently , and changes the way java script runs , such changes are :
Prevents accidental globals
No duplicates
Eliminates with
Eliminates this coercion
Safer eval()
Errors for immutables
you can also read this article for the details

"use strict"; Defines that JavaScript code should be executed in
"strict mode".
The "use strict" directive was new in ECMAScript version 5.
It is not a statement, but a literal expression, ignored by earlier
versions of JavaScript.
The purpose of "use strict" is to indicate that the code should be
executed in "strict mode".
With strict mode, you can not, for example, use undeclared variables.
All modern browsers support "use strict" except Internet Explorer 9 and lower.
Disadvantage
If a developer used a library that was in strict mode, but the developer was used to working in normal mode, they might call some actions on the library that wouldn’t work as expected.
Worse, since the developer is in normal mode, they don’t have the advantages of extra errors being thrown, so the error might fail silently.
Also, as listed above, strict mode stops you from doing certain things.
People generally think that you shouldn’t use those things in the first place, but some developers don’t like the constraint and want to use all the features of the language.
For basic example and for reference go through :
https://www.tutorialsteacher.com/javascript/javascript-strict

JavaScript was designed and implemented hastily because of the browser wars and bad management. As a result many poor design decisions, un-intuitive syntax and confusing semantics found their way into the language. Strict mode aims to amend some of these mistakes.
But fixing these mistakes without creating alternative interpretation breaks backward compatibility. So, "use strict" directive creates that alternative interpretation of the code while communicating it to the programmer.
For example, this keywords refers to the object in a method definition, like this or self in other languages.
let o = {
name: 'John Doe',
sayName: function(){
console.log(this.name);
}
};
o.sayName(); // 'John Doe'
this has no purpose outside the method context but all JavaScript functions have this keyword whether they are methods or not:
function run() {
console.log(this);
}
run(); // Window
Here this resolves to the global object which does not make sense and serves no purpose because global object is already available in the scope.
In strict mode this in a global function resolves to undefined, which is what we expect.
"use strict"
function run() {
console.log(this);
}
run(); // undefined
Some mistakes can not be fixed even in strict mode because syntax should be valid for older browsers since they ignore "strict mode" directive. This is by design.

Strict mode can prevent memory leaks.
Please check the function below written in non-strict mode:
function getname(){
name = "Stack Overflow"; // Not using var keyword
return name;
}
getname();
console.log(name); // Stack Overflow
In this function, we are using a variable called name inside the function. Internally, the compiler will first check if there is any variable declared with that particular name in that particular function scope. Since the compiler understood that there is no such variable, it will check in the outer scope. In our case, it is the global scope. Again, the compiler understood that there is also no variable declared in the global space with that name, so it creates such a variable for us in the global space. Conceptually, this variable will be created in the global scope and will be available in the entire application.
Another scenario is that, say, the variable is declared in a child function. In that case, the compiler checks the validity of that variable in the outer scope, i.e., the parent function. Only then it will check in the global space and create a variable for us there.
That means additional checks need to be done. This will affect the performance of the application.
Now let's write the same function in strict mode.
"use strict"
function getname(){
name = "Stack Overflow"; // Not using var keyword
return name;
}
getname();
console.log(name);
We will get the following error.
Uncaught ReferenceError: name is not defined
at getname (<anonymous>:3:15)
at <anonymous>:6:5
Here, the compiler throws the reference error. In strict mode, the compiler does not allow us to use the variable without declaring it. So memory leaks can be prevented. In addition, we can write more optimized code.

Strict mode eliminates errors that would be ignored in non-strict mode, thus making javascript “more secured”.
Is it considered among best practices?
Yes, It's considered part of the best practices while working with javascript to include Strict mode. This is done by adding the below line of code in your JS file.
'use strict';
in your code.
What does it mean to user agents?
Indicating that code should be interpreted in strict mode specifies to user agents like browsers that they should treat code literally as written, and throw an error if the code doesn't make sense.
For example: Consider in your .js file you have the following code:
Scenario 1: [NO STRICT MODE]
var city = "Chicago"
console.log(city) // Prints the city name, i.e. Chicago
Scenario 2: [NO STRICT MODE]
city = "Chicago"
console.log(city) // Prints the city name, i.e. Chicago
So why does the variable name is being printed in both cases?
Without strict mode turned on, user agents often go through a series of modifications to problematic code in an attempt to get it to make sense. On the surface, this can seem like a fine thing, and indeed, working outside of strict mode makes it possible for people to get their feet wet with JavaScript code without having all the details quite nailed down. However, as a developer, I don't want to leave a bug in my code, because I know it could come back and bite me later on, and I also just want to write good code. And that's where strict mode helps out.
Scenario 3: [STRICT MODE]
'use strict';
city = "Chicago"
console.log(city) // Reference Error: asignment is undeclared variable city.
Additional tip: To maintain code quality using strict mode, you don't need to write this over and again especially if you have multiple .js file. You can enforce this rule globally in eslint rules as follows:
Filename: .eslintrc.js
module.exports = {
env: {
es6: true
},
rules : {
strict: ['error', 'global'],
},
};
Okay, so what is prevented in strict mode?
Using a variable without declaring it will throw an error in strict mode. This is to prevent unintentionally creating global variables throughout your application. The example with printing Chicago covers this in particular.
Deleting a variable or a function or an argument is a no-no in strict mode.
"use strict";
function x(p1, p2) {};
delete x; // This will cause an error
Duplicating a parameter name is not allowed in strict mode.
"use strict";
function x(p1, p1) {}; // This will cause an error
Reserved words in the Javascript language are not allowed in strict mode. The words are implements interface, let, packages, private, protected, public. static, and yield
For a more comprehensive list check out the MDN documentation here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode

strict mode enables strict features in the v8 engine. Short example of some features:
You can enable it globally by writing:
'use strict'; // strict mode enabled!
Per function you just include in function:
let myfunc = () => {
'use strict'; // strict mode enabled
b = 0; // broke
}
You MUST declare a variable before using it (sane imo):
var x;
x = '0'; // ok
y = ''; // not ok
es6 features are enabled (this is browser dependent), for node v4+ this is important.
Performance, in some cases, is better.
There are more features as well, check here for more!

Motive behind strict mode syntax error when deleting an unqualified identifier?

I'm having trouble understanding why, in strict mode, a syntax error occurs when delete is used on an unqualified identifier.
In most cases, it makes sense... if you are declaring variables in the usual way with the var keyword, and then trying to use delete on them, in non-strict mode it would silently fail, so it makes sense for strict mode to fail with an error in those cases.
However, there are cases where you can't delete identifiers that are qualified:
(function() {
// "use strict";
var obj = Object.create({}, { bloop: { configurable: false } });
delete obj.bloop; // throws TypeError in strict mode, silently fails in non-strict.
console.log('bloop' in obj); // true
}());
Strict mode must do a runtime check here, because a TypeError is thrown when this is encountered. There are also cases where you can successfully delete unqualified identifiers in non-strict mode...
// "use strict";
window.bar = 6;
console.log(typeof bar); // number
delete bar; // works in non-strict, syntax error in strict!
console.log(typeof bar); // undefined
In fact, to my understanding, whether or not you can delete things (in non-strict mode) depends on the internal [[Configurable]] property, and has nothing to do with qualified identifiers. As far as I can tell, there is no way in strict mode to delete non-global variables that (as properties of the local VO) are configurable:
(function() {
// "use strict";
eval('var foo = 5;');
console.log(typeof foo); // number
delete foo; // works in non-strict, SyntaxError in strict.
console.log(typeof foo); // undefined
}());
So, my question is, what's the point of throwing a SyntaxError when using delete on an unqualified identifier, when the TypeError would throw anyway if the property is not configurable? This seems like an unnecessary restriction, and in some cases there doesn't seem to be any workaround other than not using strict mode (third example). Can anyone explain the motivation behind this decision?
Update: I just realized that I was overlooking the fact that direct eval calls have their own scope in strict mode, instead of the calling function's scope, so in the third example foo would not be defined under strict mode. Anyway, the runtime check would still catch this, but it raises a side question: Is there no way to have configurable local variables in strict mode, as we do with eval'd variable declarations in non-strict? AFAIK that was one of the few legitimate uses of eval.

You are talking about Section 11.4.1, paragraph 5.a. of the specs:
Else, ref is a Reference to an Environment Record binding, so
a. If IsStrictReference(ref) is true, throw a SyntaxError exception.
b. Let bindings be GetBase(ref).
c. Return the result of calling the DeleteBinding concrete method of bindings, providing GetReferencedName(ref) as the argument.
What you called "unqualified identifiers" is officially named "Environment Record binding".
Now, to your question. Why throw a SyntaxError when 5.c. would fail anyway? I think you answered it yourself!
Strict mode must do a runtime check here, because a TypeError is thrown when this is encountered.
That's right. But it's always better to fail fast. So, when there is a chance of detecting a SyntaxError (at parse time), that opportunity should be taken.
Why? It saves you the trouble of fixing your app if an error occurs. Think about IDEs that may show you the error right away, as opposed to hours of debugging.
Also, such restrictions may be advantageous for optimized JIT compilers.

If you want to delete object in strict mode. You have to explicitly mention about the property access. Also note that, how you call the function is important. If new operator isn't used this is undefined under use strict, and you cant use the below method.
Example:
'use strict'
function func(){
var self = this;
self.obj = {};
self.obj.x = 'y'
console.log(self.obj);
delete self.obj // works
// delete obj // doesn't work
console.log(self.obj);
}
var f = new func();
For deleting object outside of the function(closure), you will have to call like
// same code as above
delete f.obj

What does "use strict" do in JavaScript, and what is the reasoning behind it?

Recently, I ran some of my JavaScript code through Crockford's JSLint, and it gave the following error:
Problem at line 1 character 1: Missing "use strict" statement.
Doing some searching, I realized that some people add "use strict"; into their JavaScript code. Once I added the statement, the error stopped appearing. Unfortunately, Google did not reveal much of the history behind this string statement. Certainly it must have something to do with how the JavaScript is interpreted by the browser, but I have no idea what the effect would be.
So what is "use strict"; all about, what does it imply, and is it still relevant?
Do any of the current browsers respond to the "use strict"; string or is it for future use?

Update for ES6 modules
Inside native ECMAScript modules (with import and export statements) and ES6 classes, strict mode is always enabled and cannot be disabled.
Original answer
This article about Javascript Strict Mode might interest you: John Resig - ECMAScript 5 Strict Mode, JSON, and More
To quote some interesting parts:
Strict Mode is a new feature in ECMAScript 5 that allows you to place a program, or a function, in a "strict" operating context. This strict context prevents certain actions from being taken and throws more exceptions.
And:
Strict mode helps out in a couple ways:
It catches some common coding bloopers, throwing exceptions.
It prevents, or throws errors, when relatively "unsafe" actions are taken (such as gaining access to the global object).
It disables features that are confusing or poorly thought out.
Also note you can apply "strict mode" to the whole file... Or you can use it only for a specific function (still quoting from John Resig's article):
// Non-strict code...
(function(){
"use strict";
// Define your library strictly...
})();
// Non-strict code...
Which might be helpful if you have to mix old and new code ;-)
So, I suppose it's a bit like the "use strict" you can use in Perl (hence the name?): it helps you make fewer errors, by detecting more things that could lead to breakages.
Strict mode is now supported by all major browsers.

It's a new feature of ECMAScript 5. John Resig wrote up a nice summary of it.
It's just a string you put in your JavaScript files (either at the top of your file or inside of a function) that looks like this:
"use strict";
Putting it in your code now shouldn't cause any problems with current browsers as it's just a string. It may cause problems with your code in the future if your code violates the pragma. For instance, if you currently have foo = "bar" without defining foo first, your code will start failing...which is a good thing in my opinion.

The statement "use strict"; instructs the browser to use the Strict mode, which is a reduced and safer feature set of JavaScript.
List of features (non-exhaustive)
Disallows global variables. (Catches missing var declarations and typos in variable names)
Silent failing assignments will throw error in strict mode (assigning NaN = 5;)
Attempts to delete undeletable properties will throw (delete Object.prototype)
Requires all property names in an object literal to be unique (var x = {x1: "1", x1: "2"})
Function parameter names must be unique (function sum (x, x) {...})
Forbids octal syntax (var x = 023; some devs assume wrongly that a preceding zero does nothing to change the number.)
Forbids the with keyword
eval in strict mode does not introduce new variables
Forbids deleting plain names (delete x;)
Forbids binding or assignment of the names eval and arguments in any form
Strict mode does not alias properties of the arguments object with the formal parameters. (e.g. in function sum (a,b) { return arguments[0] + b;} This works because arguments[0] is bound to a and so on. ) (See examples section below to understand the difference)
arguments.callee is not supported
[Ref: Strict mode, Mozilla Developer Network]
Examples:
Strict mode code doesn't alias properties of arguments objects created within it
function show( msg ){
msg = 42;
console.log( msg ); // msg === 42
console.log( arguments[0] ); // arguments === 42
}
show( "Hey" );
// In strict mode arguments[i] does not track the value of
// the corresponding named argument, nor does a named argument track the value in the corresponding arguments[i]
function showStrict( msg ){
"use strict";
msg = 42;
console.log( msg ); // msg === 42
console.log( arguments[0] ); // arguments === "Hey"
}
showStrict( "Hey" );

If people are worried about using use strict it might be worth checking out this article:
ECMAScript 5 'Strict mode' support in browsers. What does this mean?
NovoGeek.com - Krishna's weblog
It talks about browser support, but more importantly how to deal with it safely:
function isStrictMode(){
return !this;
}
/*
returns false, since 'this' refers to global object and
'!this' becomes false
*/
function isStrictMode(){
"use strict";
return !this;
}
/*
returns true, since in strict mode the keyword 'this'
does not refer to global object, unlike traditional JS.
So here, 'this' is 'undefined' and '!this' becomes true.
*/

A word of caution, all you hard-charging programmers: applying "use strict" to existing code can be hazardous! This thing is not some feel-good, happy-face sticker that you can slap on the code to make it 'better'. With the "use strict" pragma, the browser will suddenly THROW exceptions in random places that it never threw before just because at that spot you are doing something that default/loose JavaScript happily allows but strict JavaScript abhors! You may have strictness violations hiding in seldom used calls in your code that will only throw an exception when they do eventually get run - say, in the production environment that your paying customers use!
If you are going to take the plunge, it is a good idea to apply "use strict" alongside comprehensive unit tests and a strictly configured JSHint build task that will give you some confidence that there is no dark corner of your module that will blow up horribly just because you've turned on Strict Mode. Or, hey, here's another option: just don't add "use strict" to any of your legacy code, it's probably safer that way, honestly. DEFINITELY DO NOT add "use strict" to any modules you do not own or maintain, like third party modules.
I think even though it is a deadly caged animal, "use strict" can be good stuff, but you have to do it right. The best time to go strict is when your project is greenfield and you are starting from scratch. Configure JSHint/JSLint with all the warnings and options cranked up as tight as your team can stomach, get a good build/test/assert system du jour rigged like Grunt+Karma+Chai, and only THEN start marking all your new modules as "use strict". Be prepared to cure lots of niggly errors and warnings. Make sure everyone understands the gravity by configuring the build to FAIL if JSHint/JSLint produces any violations.
My project was not a greenfield project when I adopted "use strict". As a result, my IDE is full of red marks because I don't have "use strict" on half my modules, and JSHint complains about that. It's a reminder to me about what refactoring I should do in the future. My goal is to be red mark free due to all of my missing "use strict" statements, but that is years away now.

Using 'use strict'; does not suddenly make your code better.
The JavaScript strict mode is a feature in ECMAScript 5. You can enable the strict mode by declaring this in the top of your script/function.
'use strict';
When a JavaScript engine sees this directive, it will start to interpret the code in a special mode. In this mode, errors are thrown up when certain coding practices that could end up being potential bugs are detected (which is the reasoning behind the strict mode).
Consider this example:
var a = 365;
var b = 030;
In their obsession to line up the numeric literals, the developer has inadvertently initialized variable b with an octal literal. Non-strict mode will interpret this as a numeric literal with value 24 (in base 10). However, strict mode will throw an error.
For a non-exhaustive list of specialties in strict mode, see this answer.
Where should I use 'use strict';?
In my new JavaScript application: Absolutely! Strict mode can be used as a whistleblower when you are doing something stupid with your code.
In my existing JavaScript code: Probably not! If your existing JavaScript code has statements that are prohibited in strict-mode, the application will simply break. If you want strict mode, you should be prepared to debug and correct your existing code. This is why using 'use strict'; does not suddenly make your code better.
How do I use strict mode?
Insert a 'use strict'; statement on top of your script:
// File: myscript.js
'use strict';
var a = 2;
....
Note that everything in the file myscript.js will be interpreted in strict mode.
Or, insert a 'use strict'; statement on top of your function body:
function doSomething() {
'use strict';
...
}
Everything in the lexical scope of function doSomething will be interpreted in strict mode. The word lexical scope is important here. For example, if your strict code calls a function of a library that is not strict, only your code is executed in strict mode, and not the called function. See this answer for a better explanation.
What things are prohibited in strict mode?
I found a nice article describing several things that are prohibited in strict mode (note that this is not an exhaustive list):
Scope
Historically, JavaScript has been confused about how functions
are scoped. Sometimes they seem to be statically scoped, but some
features make them behave like they are dynamically scoped. This is
confusing, making programs difficult to read and understand.
Misunderstanding causes bugs. It also is a problem for performance.
Static scoping would permit variable binding to happen at compile
time, but the requirement for dynamic scope means the binding must be
deferred to runtime, which comes with a significant performance
penalty.
Strict mode requires that all variable binding be done statically.
That means that the features that previously required dynamic binding
must be eliminated or modified. Specifically, the with statement is
eliminated, and the eval function’s ability to tamper with the
environment of its caller is severely restricted.
One of the benefits of strict code is that tools like YUI Compressor
can do a better job when processing it.
Implied Global Variables
JavaScript has implied global variables. If
you do not explicitly declare a variable, a global variable is
implicitly declared for you. This makes programming easier for
beginners because they can neglect some of their basic housekeeping
chores. But it makes the management of larger programs much more
difficult and it significantly degrades reliability. So in strict
mode, implied global variables are no longer created. You should
explicitly declare all of your variables.
Global Leakage
There are a number of situations that could cause this
to be bound to the global object. For example, if you forget to
provide the new prefix when calling a constructor function, the
constructor's this will be bound unexpectedly to the global object, so
instead of initializing a new object, it will instead be silently
tampering with global variables. In these situations, strict mode will
instead bind this to undefined, which will cause the constructor to
throw an exception instead, allowing the error to be detected much
sooner.
Noisy Failure
JavaScript has always had read-only properties, but you
could not create them yourself until ES5’s Object.createProperty
function exposed that capability. If you attempted to assign a value
to a read-only property, it would fail silently. The assignment would
not change the property’s value, but your program would proceed as
though it had. This is an integrity hazard that can cause programs to
go into an inconsistent state. In strict mode, attempting to change a
read-only property will throw an exception.
Octal
The octal (or base 8) representation of numbers was extremely
useful when doing machine-level programming on machines whose word
sizes were a multiple of 3. You needed octal when working with the CDC
6600 mainframe, which had a word size of 60 bits. If you could read
octal, you could look at a word as 20 digits. Two digits represented
the op code, and one digit identified one of 8 registers. During the
slow transition from machine codes to high level languages, it was
thought to be useful to provide octal forms in programming languages.
In C, an extremely unfortunate representation of octalness was
selected: Leading zero. So in C, 0100 means 64, not 100, and 08 is an
error, not 8. Even more unfortunately, this anachronism has been
copied into nearly all modern languages, including JavaScript, where
it is only used to create errors. It has no other purpose. So in
strict mode, octal forms are no longer allowed.
Et cetera
The arguments pseudo array becomes a little bit more
array-like in ES5. In strict mode, it loses its callee and caller
properties. This makes it possible to pass your arguments to untrusted
code without giving up a lot of confidential context. Also, the
arguments property of functions is eliminated.
In strict mode, duplicate keys in a function literal will produce a
syntax error. A function can’t have two parameters with the same name.
A function can’t have a variable with the same name as one of its
parameters. A function can’t delete its own variables. An attempt to
delete a non-configurable property now throws an exception. Primitive
values are not implicitly wrapped.
Reserved words for future JavaScript versions
ECMAScript 5 adds a list of reserved words. If you use them as variables or arguments, strict mode will throw an error. The reserved words are:
implements, interface, let, package, private, protected, public, static, and yield
Further Reading
Strict Mode - JavaScript | MDN
Browser support for strict mode
Transitioning to strict mode

I strongly recommend every developer to start using strict mode now. There are enough browsers supporting it that strict mode will legitimately help save us from errors we didn’t even know were in your code.
Apparently, at the initial stage there will be errors we have never encountered before. To get the full benefit, we need to do proper testing after switching to strict mode to make sure we have caught everything. Definitely we don’t just throw use strict in our code and assume there are no errors. So the churn is that it’s time to start using this incredibly useful language feature to write better code.
For example,
var person = {
name : 'xyz',
position : 'abc',
fullname : function () { "use strict"; return this.name; }
};
JSLint is a debugger written by Douglas Crockford. Simply paste in your script, and it’ll quickly scan for any noticeable issues and errors in your code.

I would like to offer a somewhat more founded answer complementing the other answers. I was hoping to edit the most popular answer, but failed. I tried to make it as comprehensive and complete as I could.
You can refer to the MDN documentation for more information.
"use strict" a directive introduced in ECMAScript 5.
Directives are similar to statements, yet different.
use strict does not contain key words: The directive is a simple expression statement, which consists of a special string literal (in single or double quotes). JavaScript engines, that do not implement ECMAScript 5, merely see an expression statement without side effects. It is expected that future versions of ECMAScript standards introduce use as a real key word; the quotes would thereby become obsolete.
use strict can be used only at the beginning of a script or of a function, i.e. it must precede every other (real) statement. It does not have to be the first instruction in a script of function: it can be preceded by other statement expressions that consist of string literals ( and JavaScript implementations can treat them as implementation specific directives). String literals statements, which follow a first real statement (in a script or function) are simple expression statements. Interpreters must not interpret them as directives and they have no effect.
The use strict directive indicates that the following code (in a script or a function) is strict code.
The code in the highest level of a script (code that is not in a function) is considered strict code when the script contains a use strict directive.
The content of a function is considered strict code when the function itself is defined in a strict code or when the function contains a use strict directive.
Code that is passed to an eval() method is considered strict code when eval() was called from a strict code or contains the use strict directive itself.
The strict mode of ECMAScript 5 is a restricted subset of the JavaScript language, which eliminates relevant deficits of the language and features more stringent error checking and higher security. The following lists the differences between strict mode and normal mode (of which the first three are particularly important):
You cannot use the with-statement in strict mode.
In strict mode all variables have to be declared: if you assign a value to an identifier that has not been declared as variable, function, function parameter, catch-clause parameter or property of the global Object, then you will get a ReferenceError. In normal mode the identifier is implicitly declared as a global variable (as a property of the global Object)
In strict mode the keyword this has the value undefined in functions that were invoked as functions (not as methods). (In normal mode this always points to the global Object). This difference can be used to test if an implementation supports the strict mode:
var hasStrictMode = (function() { "use strict"; return this===undefined }());
Also when a function is invoked with call() or apply in strict mode, then this is exactly the value of the first argument of the call()or apply() invocation. (In normal mode null and undefined are replaced by the global Object and values, which are not objects, are cast into objects.)
In strict mode you will get a TypeError, when you try to assign to readonly properties or to define new properties for a non extensible object. (In normal mode both simply fail without error message.)
In strict mode, when passing code to eval(), you cannot declare or define variables or functions in the scope of the caller (as you can do it in normal mode). Instead, a new scope is created for eval() and the variables and functions are within that scope. That scope is destroyed after eval() finishes execution.
In strict mode the arguments-object of a function contains a static copy of the values, which are passed to that function. In normal mode the arguments-object has a somewhat "magical" behaviour: The elements of the array and the named function parameters reference both the same value.
In strict mode you will get a SyntaxError when the delete operator is followed by a non qualified identifier (a variable, function or function parameter). In normal mode the delete expression would do nothing and is evaluated to false.
In strict mode you will get a TypeError when you try to delete a non configurable property. (In normal mode the attempt simply fails and the delete expression is evaluated to false).
In strict mode it is considered a syntactical error when you try to define several properties with the same name for an object literal. (In normal mode there is no error.)
In strict mode it is considered a syntactical error when a function declaration has multiple parameters with the same name. (In normal mode there is no error.)
In strict mode octal literals are not allowed (these are literals that start with 0. (In normal mode some implementations do allow octal literals.)
In strict mode the identifiers eval and arguments are treated like keywords. You cannot change their value, cannot assign a value to them, and you cannot use them as names for variables, functions, function parameters or identifiers of a catch block.
In strict mode are more restrictions on the possibilities to examine the call stack. arguments.caller and arguments.callee cause a TypeError in a function in strict mode. Furthermore, some caller- and arguments properties of functions in strict mode cause a TypeError when you try to read them.

My two cents:
One of the goals of strict mode is to allow for faster debugging of issues. It helps the developers by throwing exception when certain wrong things occur that can cause silent & strange behaviour of your webpage. The moment we use use strict, the code will throw out errors which helps developer to fix it in advance.
Few important things which I have learned after using use strict :
Prevents Global Variable Declaration:
"use strict";
var tree1Data = { name: 'Banana Tree',age: 100,leafCount: 100000};
function Tree(typeOfTree) {
var age;
var leafCount;
age = typeOfTree.age;
leafCount = typeOfTree.leafCount;
nameoftree = typeOfTree.name;
};
var tree1 = new Tree(tree1Data);
console.log(window);
Now,this code creates nameoftree in global scope which could be accessed using window.nameoftree. When we implement use strict the code would throw error.
Uncaught ReferenceError: nameoftree is not defined
Eliminates with statement :
with statements can't be minified using tools like uglify-js. They're also deprecated and removed from future JavaScript versions.
Sample:
"use strict";
var tree1Data = {
name: 'Banana Tree',
age: 100,
leafCount: 100000
};
function Tree(typeOfTree) {
var age;
var leafCount;
age = typeOfTree.age;
leafCount = typeOfTree.leafCount;
// nameoftree = typeOfTree.name;
for (var i = 0; i < 2; ++i) {
// let(leafCount = i) { /*do something*/ }
}
for (var i = 0; i < 2; ++i) {
with(leafCount = i) { /*do something*/ }
}
};
var tree1 = new Tree(tree1Data);
console.log(window);
Prevents Duplicates :
When we have duplicate property, it throws an exception
Uncaught SyntaxError: Duplicate data property in object literal not
allowed in strict mode
"use strict";
var tree1Data = {
name: 'Banana Tree',
age: 100,
leafCount: 100000,
name:'Banana Tree'
};
There are few more but I need to gain more knowledge on that.

If you use a browser released in the last year or so then it most likely supports JavaScript Strict mode. Only older browsers around before ECMAScript 5 became the current standard don't support it.
The quotes around the command make sure that the code will still work in older browsers as well (although the things that generate a syntax error in strict mode will generally just cause the script to malfunction in some hard to detect way in those older browsers).

When adding "use strict";, the following cases will throw a SyntaxError before the script is executing:
Paving the way for future ECMAScript versions, using one of the newly reserved keywords (in prevision for ECMAScript 6): implements, interface, let, package, private, protected, public, static, and yield.
Declaring function in blocks
if(a<b){ function f(){} }
Octal syntax
var n = 023;
this point to the global object.
function f() {
"use strict";
this.a = 1;
};
f();
Declaring twice the same name for a property name in an object literal
{a: 1, b: 3, a: 7}
This is no longer the case in ECMAScript 6 (bug 1041128).
Declaring two function arguments with the same name function
f(a, b, b){}
Setting a value to an undeclared variable
function f(x){
"use strict";
var a = 12;
b = a + x*35; // error!
}
f();
Using delete on a variable name delete myVariable;
Using eval or arguments as variable or function argument name
"use strict";
arguments++;
var obj = { set p(arguments) { } };
try { } catch (arguments) { }
function arguments() { }
Sources:
Transitioning to strict mode on MDN
Strict mode on MDN
JavaScript’s Strict Mode and Why You Should Use It on Colin J. Ihrig's blog (archived version)

Strict mode makes several changes to normal JavaScript semantics:
eliminates some JavaScript silent errors by changing them
to throw errors.
fixes mistakes that make it difficult for JavaScript
engines to perform optimizations.
prohibits some syntax likely to be defined in future
versions of ECMAScript.
for more information vistit Strict Mode- Javascript

"Use Strict"; is an insurance that programmer will not use the loose or the bad properties of JavaScript. It is a guide, just like a ruler will help you make straight lines. "Use Strict" will help you do "Straight coding".
Those that prefer not to use rulers to do their lines straight usually end up in those pages asking for others to debug their code.
Believe me. The overhead is negligible compared to poorly designed code. Doug Crockford, who has been a senior JavaScript developer for several years, has a very interesting post here. Personally, I like to return to his site all the time to make sure I don't forget my good practice.
Modern JavaScript practice should always evoke the "Use Strict"; pragma. The only reason that the ECMA Group has made the "Strict" mode optional is to permit less experienced coders access to JavaScript and give then time to adapt to the new and safer coding practices.

Including use strict in the beginning of your all sensitive JavaScript files from this point is a small way to be a better JavaScript programmer and avoid random variables becoming global and things change silently.

Quoting from w3schools:
The "use strict" Directive
The "use strict" directive is new in JavaScript 1.8.5 (ECMAScript
version 5).
It is not a statement, but a literal expression, ignored by earlier
versions of JavaScript.
The purpose of "use strict" is to indicate that the code should be
executed in "strict mode".
With strict mode, you can not, for example, use undeclared variables.
Why Strict Mode?
Strict mode makes it easier to write "secure" JavaScript.
Strict mode changes previously accepted "bad syntax" into real errors.
As an example, in normal JavaScript, mistyping a variable name creates
a new global variable. In strict mode, this will throw an error,
making it impossible to accidentally create a global variable.
In normal JavaScript, a developer will not receive any error feedback
assigning values to non-writable properties.
In strict mode, any assignment to a non-writable property, a
getter-only property, a non-existing property, a non-existing
variable, or a non-existing object, will throw an error.
Please refer to http://www.w3schools.com/js/js_strict.asp to know more

"use strict" makes JavaScript code to run in strict mode, which basically means everything needs to be defined before use. The main reason for using strict mode is to avoid accidental global uses of undefined methods.
Also in strict mode, things run faster, some warnings or silent warnings throw fatal errors, it's better to always use it to make a neater code.
"use strict" is widely needed to be used in ECMA5, in ECMA6 it's part of JavaScript by default, so it doesn't need to be added if you're using ES6.
Look at these statements and examples from MDN:
The "use strict" Directive The "use strict" directive is new in
JavaScript 1.8.5 (ECMAScript version 5). It is not a statement, but a
literal expression, ignored by earlier versions of JavaScript. The
purpose of "use strict" is to indicate that the code should be
executed in "strict mode". With strict mode, you can not, for example,
use undeclared variables.
Examples of using "use strict":
Strict mode for functions: Likewise, to invoke strict mode for a
function, put the exact statement "use strict"; (or 'use strict';) in
the function's body before any other statements.
1) strict mode in functions
function strict() {
// Function-level strict mode syntax
'use strict';
function nested() { return 'And so am I!'; }
return "Hi! I'm a strict mode function! " + nested();
}
function notStrict() { return "I'm not strict."; }
console.log(strict(), notStrict());
2) whole-script strict mode
'use strict';
var v = "Hi! I'm a strict mode script!";
console.log(v);
3) Assignment to a non-writable global
'use strict';
// Assignment to a non-writable global
var undefined = 5; // throws a TypeError
var Infinity = 5; // throws a TypeError
// Assignment to a non-writable property
var obj1 = {};
Object.defineProperty(obj1, 'x', { value: 42, writable: false });
obj1.x = 9; // throws a TypeError
// Assignment to a getter-only property
var obj2 = { get x() { return 17; } };
obj2.x = 5; // throws a TypeError
// Assignment to a new property on a non-extensible object.
var fixed = {};
Object.preventExtensions(fixed);
fixed.newProp = 'ohai'; // throws a TypeError
You can read more on MDN.

There's a good talk by some people who were on the ECMAScript committee: Changes to JavaScript, Part 1: ECMAScript 5" about how incremental use of the "use strict" switch allows JavaScript implementers to clean up a lot of the dangerous features of JavaScript without suddenly breaking every website in the world.
Of course it also talks about just what a lot of those misfeatures are (were) and how ECMAScript 5 fixes them.

Small examples to compare:
Non-strict mode:
for (i of [1,2,3]) console.log(i)
// output:
// 1
// 2
// 3
Strict mode:
'use strict';
for (i of [1,2,3]) console.log(i)
// output:
// Uncaught ReferenceError: i is not defined
Non-strict mode:
String.prototype.test = function () {
console.log(typeof this === 'string');
};
'a'.test();
// output
// false
String.prototype.test = function () {
'use strict';
console.log(typeof this === 'string');
};
'a'.test();
// output
// true

Note that use strict was introduced in EcmaScript 5 and was kept since then.
Below are the conditions to trigger strict mode in ES6 and ES7:
Global code is strict mode code if it begins with a Directive Prologue that contains a Use Strict Directive (see 14.1.1).
Module code is always strict mode code.
All parts of a ClassDeclaration or a ClassExpression are strict mode code.
Eval code is strict mode code if it begins with a Directive Prologue that contains a Use Strict Directive or if the call to eval is a direct eval (see 12.3.4.1) that is contained in strict mode code.
Function code is strict mode code if the associated FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, MethodDefinition, or ArrowFunction is contained in strict mode code or if the code that produces the value of the function’s [[ECMAScriptCode]] internal slot begins with a Directive Prologue that contains a Use Strict Directive.
Function code that is supplied as the arguments to the built-in Function and Generator constructors is strict mode code if the last argument is a String that when processed is a FunctionBody that begins with a Directive Prologue that contains a Use Strict Directive.

The main reasons why developers should use "use strict" are:
Prevents accidental declaration of global variables.Using "use strict()" will make sure that variables are declared with var before use.
Eg:
function useStrictDemo(){
'use strict';
//works fine
var a = 'No Problem';
//does not work fine and throws error
k = "problem"
//even this will throw error
someObject = {'problem': 'lot of problem'};
}
N.B: The "use strict" directive is only recognized at the beginning of a script or a function.
The string "arguments" cannot be used as a variable:
"use strict";
var arguments = 3.14; // This will cause an error
Will restrict uses of keywords as variables. Trying to use them will throw errors.
In short will make your code less error prone and in turn will make you write good code.
To read more about it you can refer here.

use strict is a way to make your code safer, because you can't use dangerous features that can work not as you expect. And, as was written before, it makes code more strict.

JavaScript “strict” mode was introduced in ECMAScript 5.
(function() {
"use strict";
your code...
})();
Writing "use strict"; at the very top of your JS file turns on strict
syntax checking. It does the following tasks for us:
shows an error if you try to assign to an undeclared variable
stops you from overwriting key JS system libraries
forbids some unsafe or error-prone language features
use strict also works inside of individual functions. It is always a better practice to include use strict in your code.
Browser compatibility issue:
The "use" directives are meant to be backwards-compatible. Browsers that do not support them will just see a string literal that isn't referenced further. So, they will pass over it and move on.

"use strict"; is the ECMA effort to make JavaScript a little bit more robust. It brings in JS an attempt to make it at least a little "strict" (other languages implement strict rules since the 90s). It actually "forces" JavaScript developers to follow some sort of coding best practices.
Still, JavaScript is very fragile. There is no such thing as typed variables, typed methods, etc.
I strongly recommend JavaScript developers to learn a more robust language such as Java or ActionScript3, and implement the same best practices in your JavaScript code, it will work better and be easier to debug.

Normally, JavaScript does not follow strict rules, hence increasing chances of errors. After using "use strict", the JavaScript code should follow strict set of rules as in other programming languages such as use of terminators, declaration before initialization, etc.
If "use strict" is used, the code should be written by following a strict set of rules, hence decreasing the chances of errors and ambiguities.

Use Strict is used to show common and repeated errors so that it is handled differently , and changes the way java script runs , such changes are :
Prevents accidental globals
No duplicates
Eliminates with
Eliminates this coercion
Safer eval()
Errors for immutables
you can also read this article for the details

"use strict"; Defines that JavaScript code should be executed in
"strict mode".
The "use strict" directive was new in ECMAScript version 5.
It is not a statement, but a literal expression, ignored by earlier
versions of JavaScript.
The purpose of "use strict" is to indicate that the code should be
executed in "strict mode".
With strict mode, you can not, for example, use undeclared variables.
All modern browsers support "use strict" except Internet Explorer 9 and lower.
Disadvantage
If a developer used a library that was in strict mode, but the developer was used to working in normal mode, they might call some actions on the library that wouldn’t work as expected.
Worse, since the developer is in normal mode, they don’t have the advantages of extra errors being thrown, so the error might fail silently.
Also, as listed above, strict mode stops you from doing certain things.
People generally think that you shouldn’t use those things in the first place, but some developers don’t like the constraint and want to use all the features of the language.
For basic example and for reference go through :
https://www.tutorialsteacher.com/javascript/javascript-strict

JavaScript was designed and implemented hastily because of the browser wars and bad management. As a result many poor design decisions, un-intuitive syntax and confusing semantics found their way into the language. Strict mode aims to amend some of these mistakes.
But fixing these mistakes without creating alternative interpretation breaks backward compatibility. So, "use strict" directive creates that alternative interpretation of the code while communicating it to the programmer.
For example, this keywords refers to the object in a method definition, like this or self in other languages.
let o = {
name: 'John Doe',
sayName: function(){
console.log(this.name);
}
};
o.sayName(); // 'John Doe'
this has no purpose outside the method context but all JavaScript functions have this keyword whether they are methods or not:
function run() {
console.log(this);
}
run(); // Window
Here this resolves to the global object which does not make sense and serves no purpose because global object is already available in the scope.
In strict mode this in a global function resolves to undefined, which is what we expect.
"use strict"
function run() {
console.log(this);
}
run(); // undefined
Some mistakes can not be fixed even in strict mode because syntax should be valid for older browsers since they ignore "strict mode" directive. This is by design.

Strict mode can prevent memory leaks.
Please check the function below written in non-strict mode:
function getname(){
name = "Stack Overflow"; // Not using var keyword
return name;
}
getname();
console.log(name); // Stack Overflow
In this function, we are using a variable called name inside the function. Internally, the compiler will first check if there is any variable declared with that particular name in that particular function scope. Since the compiler understood that there is no such variable, it will check in the outer scope. In our case, it is the global scope. Again, the compiler understood that there is also no variable declared in the global space with that name, so it creates such a variable for us in the global space. Conceptually, this variable will be created in the global scope and will be available in the entire application.
Another scenario is that, say, the variable is declared in a child function. In that case, the compiler checks the validity of that variable in the outer scope, i.e., the parent function. Only then it will check in the global space and create a variable for us there.
That means additional checks need to be done. This will affect the performance of the application.
Now let's write the same function in strict mode.
"use strict"
function getname(){
name = "Stack Overflow"; // Not using var keyword
return name;
}
getname();
console.log(name);
We will get the following error.
Uncaught ReferenceError: name is not defined
at getname (<anonymous>:3:15)
at <anonymous>:6:5
Here, the compiler throws the reference error. In strict mode, the compiler does not allow us to use the variable without declaring it. So memory leaks can be prevented. In addition, we can write more optimized code.

Strict mode eliminates errors that would be ignored in non-strict mode, thus making javascript “more secured”.
Is it considered among best practices?
Yes, It's considered part of the best practices while working with javascript to include Strict mode. This is done by adding the below line of code in your JS file.
'use strict';
in your code.
What does it mean to user agents?
Indicating that code should be interpreted in strict mode specifies to user agents like browsers that they should treat code literally as written, and throw an error if the code doesn't make sense.
For example: Consider in your .js file you have the following code:
Scenario 1: [NO STRICT MODE]
var city = "Chicago"
console.log(city) // Prints the city name, i.e. Chicago
Scenario 2: [NO STRICT MODE]
city = "Chicago"
console.log(city) // Prints the city name, i.e. Chicago
So why does the variable name is being printed in both cases?
Without strict mode turned on, user agents often go through a series of modifications to problematic code in an attempt to get it to make sense. On the surface, this can seem like a fine thing, and indeed, working outside of strict mode makes it possible for people to get their feet wet with JavaScript code without having all the details quite nailed down. However, as a developer, I don't want to leave a bug in my code, because I know it could come back and bite me later on, and I also just want to write good code. And that's where strict mode helps out.
Scenario 3: [STRICT MODE]
'use strict';
city = "Chicago"
console.log(city) // Reference Error: asignment is undeclared variable city.
Additional tip: To maintain code quality using strict mode, you don't need to write this over and again especially if you have multiple .js file. You can enforce this rule globally in eslint rules as follows:
Filename: .eslintrc.js
module.exports = {
env: {
es6: true
},
rules : {
strict: ['error', 'global'],
},
};
Okay, so what is prevented in strict mode?
Using a variable without declaring it will throw an error in strict mode. This is to prevent unintentionally creating global variables throughout your application. The example with printing Chicago covers this in particular.
Deleting a variable or a function or an argument is a no-no in strict mode.
"use strict";
function x(p1, p2) {};
delete x; // This will cause an error
Duplicating a parameter name is not allowed in strict mode.
"use strict";
function x(p1, p1) {}; // This will cause an error
Reserved words in the Javascript language are not allowed in strict mode. The words are implements interface, let, packages, private, protected, public. static, and yield
For a more comprehensive list check out the MDN documentation here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode

strict mode enables strict features in the v8 engine. Short example of some features:
You can enable it globally by writing:
'use strict'; // strict mode enabled!
Per function you just include in function:
let myfunc = () => {
'use strict'; // strict mode enabled
b = 0; // broke
}
You MUST declare a variable before using it (sane imo):
var x;
x = '0'; // ok
y = ''; // not ok
es6 features are enabled (this is browser dependent), for node v4+ this is important.
Performance, in some cases, is better.
There are more features as well, check here for more!