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How come no 'var' needed inside js function? [duplicate]

This question already has answers here:
What is the purpose of the var keyword and when should I use it (or omit it)?
(19 answers)
Closed 7 years ago.
Is "var" optional?
myObj = 1;
same as ?
var myObj = 1;
I found they both work from my test, I assume var is optional. Is that right?

They mean different things.
If you use var the variable is declared within the scope you are in (e.g. of the function). If you don't use var, the variable bubbles up through the layers of scope until it encounters a variable by the given name or the global object (window, if you are doing it in the browser), where it then attaches. It is then very similar to a global variable. However, it can still be deleted with delete (most likely by someone else's code who also failed to use var). If you use var in the global scope, the variable is truly global and cannot be deleted.
This is, in my opinion, one of the most dangerous issues with javascript, and should be deprecated, or at least raise warnings over warnings. The reason is, it's easy to forget var and have by accident a common variable name bound to the global object. This produces weird and difficult to debug behavior.

This is one of the tricky parts of Javascript, but also one of its core features. A variable declared with var "begins its life" right where you declare it. If you leave out the var, it's like you're talking about a variable that you have used before.
var foo = 'first time use';
foo = 'second time use';
With regards to scope, it is not true that variables automatically become global. Rather, Javascript will traverse up the scope chain to see if you have used the variable before. If it finds an instance of a variable of the same name used before, it'll use that and whatever scope it was declared in. If it doesn't encounter the variable anywhere it'll eventually hit the global object (window in a browser) and will attach the variable to it.
var foo = "I'm global";
var bar = "So am I";
function () {
var foo = "I'm local, the previous 'foo' didn't notice a thing";
var baz = "I'm local, too";
function () {
var foo = "I'm even more local, all three 'foos' have different values";
baz = "I just changed 'baz' one scope higher, but it's still not global";
bar = "I just changed the global 'bar' variable";
xyz = "I just created a new global variable";
}
}
This behavior is really powerful when used with nested functions and callbacks. Learning about what functions are and how scope works is the most important thing in Javascript.

Nope, they are not equivalent.
With myObj = 1; you are using a global variable.
The latter declaration create a variable local to the scope you are using.
Try the following code to understand the differences:
external = 5;
function firsttry() {
var external = 6;
alert("first Try: " + external);
}
function secondtry() {
external = 7;
alert("second Try: " + external);
}
alert(external); // Prints 5
firsttry(); // Prints 6
alert(external); // Prints 5
secondtry(); // Prints 7
alert(external); // Prints 7
The second function alters the value of the global variable "external", but the first function doesn't.

There's a bit more to it than just local vs global. Global variables created with var are different than those created without. Consider this:
var foo = 1; // declared properly
bar = 2; // implied global
window.baz = 3; // global via window object
Based on the answers so far, these global variables, foo, bar, and baz are all equivalent. This is not the case. Global variables made with var are (correctly) assigned the internal [[DontDelete]] property, such that they cannot be deleted.
delete foo; // false
delete bar; // true
delete baz; // true
foo; // 1
bar; // ReferenceError
baz; // ReferenceError
This is why you should always use var, even for global variables.

There's so much confusion around this subject, and none of the existing answers cover everything clearly and directly. Here are some examples with comments inline.
//this is a declaration
var foo;
//this is an assignment
bar = 3;
//this is a declaration and an assignment
var dual = 5;
A declaration sets a DontDelete flag. An assignment does not.
A declaration ties that variable to the current scope.
A variable assigned but not declared will look for a scope to attach itself to. That means it will traverse up the food-chain of scope until a variable with the same name is found. If none is found, it will be attached to the top-level scope (which is commonly referred to as global).
function example(){
//is a member of the scope defined by the function example
var foo;
//this function is also part of the scope of the function example
var bar = function(){
foo = 12; // traverses scope and assigns example.foo to 12
}
}
function something_different(){
foo = 15; // traverses scope and assigns global.foo to 15
}
For a very clear description of what is happening, this analysis of the delete function covers variable instantiation and assignment extensively.

var is optional. var puts a variable in local scope. If a variable is defined without var, it is in global scope and not deletable.
edit
I thought that the non-deletable part was true at some point in time with a certain environment. I must have dreamed it.

Check out this Fiddle: http://jsfiddle.net/GWr6Z/2/
function doMe(){
a = "123"; // will be global
var b = "321"; // local to doMe
alert("a:"+a+" -- b:"+b);
b = "something else"; // still local (not global)
alert("a:"+a+" -- b:"+b);
};
doMe()
alert("a:"+a+" -- b:"+b); // `b` will not be defined, check console.log

They are not the same.
Undeclared variable (without var) are treated as properties of the global object. (Usually the window object, unless you're in a with block)
Variables declared with var are normal local variables, and are not visible outside the function they're declared in. (Note that Javascript does not have block scope)
Update: ECMAScript 2015
let was introduced in ECMAScript 2015 to have block scope.

The var keyword in Javascript is there for a purpose.
If you declare a variable without the var keyword, like this:
myVar = 100;
It becomes a global variable that can be accessed from any part of your script. If you did not do it intentionally or are not aware of it, it can cause you pain if you re-use the variable name at another place in your javascript.
If you declare the variable with the var keyword, like this:
var myVar = 100;
It is local to the scope ({] - braces, function, file, depending on where you placed it).
This a safer way to treat variables. So unless you are doing it on purpose try to declare variable with the var keyword and not without.

Consider this question asked at StackOverflow today:
Simple Javascript question
A good test and a practical example is what happens in the above scenario...
The developer used the name of the JavaScript function in one of his variables.
What's the problem with the code?
The code only works the first time the user clicks the button.
What's the solution?
Add the var keyword before the variable name.

Var doesn't let you, the programmer, declare a variable because Javascript doesn't have variables. Javascript has objects. Var declares a name to an undefined object, explicitly. Assignment assigns a name as a handle to an object that has been given a value.
Using var tells the Javacript interpreter two things:
not to use delegation reverse traversal look up value for the name, instead use this one
not to delete the name
Omission of var tells the Javacript interpreter to use the first-found previous instance of an object with the same name.
Var as a keyword arose from a poor decision by the language designer much in the same way that Javascript as a name arose from a poor decision.
ps. Study the code examples above.

Everything about scope aside, they can be used differently.
console.out(var myObj=1);
//SyntaxError: Unexpected token var
console.out(myObj=1);
//1
Something something statement vs expression

No, it is not "required", but it might as well be as it can cause major issues down the line if you don't. Not defining a variable with var put that variable inside the scope of the part of the code it's in. If you don't then it isn't contained in that scope and can overwrite previously defined variables with the same name that are outside the scope of the function you are in.

I just found the answer from a forum referred by one of my colleague. If you declare a variable outside a function, it's always global. No matter if you use var keyword or not. But, if you declare the variable inside a function, it has a big difference. Inside a function, if you declare the variable using var keyword, it will be local, but if you declare the variable without var keyword, it will be global. It can overwrite your previously declared variables. - See more at: http://forum.webdeveloperszone.com/question/what-is-the-difference-between-using-var-keyword-or-not-using-var-during-variable-declaration/#sthash.xNnLrwc3.dpuf

Replace a global variable temporarily in a local scope

Before actually asking anything, I'll go ahead and say this is a theoretical question; however, it might be implemented on a website later on.
Anyway, I have a variable, any variable. Let's say it's a and its scope is global. Now, for a specific function, I want to set that variable's value to something other than it's global value, but based on it, and without changing its value globally. For example:
a = {something: "Safe", other: "Foo"}
function hello(){
var a = a.other; // Foo
a.something; // Undefined
}
a.something; // Safe
a.other; // Foo
The issue with the above code is that when I define var a in the function, it will have already cleared the value of the global a locally before setting it; in other words, it would return something like Can't access property [other] of undefined [a].
Again, a should still be a (so using another variable name is not an option, or at least not the ideal one). In fact, the global a should not be accessible from the function hello.
Edit: window will also be overwritten with null, regarding Milan Jaric's answer.
Thanks in advance!

Every global can be accessed using window object, with a little changing your code here is example
a = {something: "Safe", other: "Foo"}
function hello(){
var a = window.a.other; // Foo
console.log(window.a.something); // Safe
}
a.something; // Safe
a.other; // Foo
hello();
or
a = {something: "Safe", other: "Foo"}
function hello(){
var a = this.a.other; // Foo
delete a;
console.log(this.a.something); // Safe
}
a.something; // Safe
a.other; // Foo
hello();

This is what I was looking for...now, before you think I had the answer before I asked, I didn't, I was only able to reach a tangible solution based on Milan Jaric's answer (thanks btw).
a = {something: "Safe", other: "Foo"}
function hello(b){
var window = null;
var a = b; // a.other;
a.something; // Undefined
}
a.something // Safe
a.other // Foo
hello(a.other)
(I never really said what could or couldn't go outside the function).

Let's say it's a and its scope is global.
You mean "a is a global variable".
... for a specific function, I want to set that variable's value to
something other than it's global value, but based on it, and without
changing its value globally.
Impossible. You can create a variable with the same name that is on a scope chain, however you can't conditionally create properties of variable objects (i.e. the objects used for identifier resolution on the scope chain). You can only declare local varaibles, which means they exist before any code is run and so can't be conditional, or you can assign directly to an undeclared identifier at which point it becomes a global variable.
[snipped code]
The issue with the above code is that when I define var a in the
function, it will have already cleared the value of the global a
locally before setting it;
The code doesn't in any way "clear" the value of a. It creates a local variable a so that the identifier a will resolve to that variable, not to the global a. To differentiate betweent the two, you can access the global a as a property of the global object:
var a = 'whatever';
var myFunction = (function(global) {
return function() {
var a; // local a
global.a; // global a
}
}(this));
Again, a should still be a (so using another variable name is not an
option, or at least not the ideal one). In fact, the global a should
not be accessible from the function hello.
Impossible, though it might be almost possible in ES5 strict mode provided the code attempting to access the global a is inside another function and can't get a reference to the global object.
But I don't think you can guarantee that.

When do I use var? [duplicate]

This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
JavaScript Variable Scope
My understanding it that with in a function if I use var then I have a local variable. If I do not delcare var I now have a global variable.
But what about oustide of functions, what effect does var have?

First of all, it's generally bad practice to use code outside of functions. If nothing else, wrap your code in anonymous functions:
(function(){
// code
})();
As for what effect var has, it "declares" a variable:
var foo;
alert(foo); // undefined;
vs:
alert(foo); // error: foo is not defined
The reason for this is that the above code is functionally identical to:
alert(window.foo);
without declaring the variable with var, you get a lookup error, the same as trying to access the property of any object that doesn't exist.
Note that one of the oddities of var is that all declarations are pulled to the top of the script, so this will work as well:
alert(foo); // undefined
var foo;
You will also have access to your variable in the window object (though you will also have this by setting the variable without var, e.g. just foo=42):
var foo;
for(var key in window){
// one of these keys will be 'foo'
}

It is good practice to always use var. Strictly speaking when you are already in the global scope you don't need to but for the sake of code maintainability you should.
Say you have:
foo = 'bar';
But later you decide you want to move this code into a function:
function doSomething() {
foo = 'bar'; // oops forgot to add var
}
If you forget to add a var statement you've just created an implicit global. Now if you create a new object in the global scope that is named foo they will cause conflicts with one another.
function doSomething() {
foo = 'bar'; // Implicit global
}
foo = 'baz';
doSomething();
console.log(foo); // Returns 'bar', not 'baz'
This kind of error is particularly insidious when you forget to use var on something like i in a for loop. Learning to use JSLint can help to avoid these and other problematic logic or syntax errors.

Your question is answered in https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/var
Using var outside a function is
optional; assigning a value to an
undeclared variable implicitly
declares it as a global variable.
However, it is recommended to always
use var, and it is necessary within
functions in the following situations:
If a variable in a scope containing the function (including the global scope) has the same name.
If recursive or multiple functions use variables with the same name and> intend those variables to be local.
Failure to declare the variable in
these cases will very likely lead to
unexpected results.

I believe using var outside of a function works the same as not using var: you get a global. The exception to this would be if you are in a class or some other namespace structure, in which it will still define a variable in that locale

i believe you want to create a var whenever you are initializing a variable. As i've coded, when ever i need to initialize a variable, i start it with var. If you declare a variable without the word var, it's always global. If you declare a variable with var, inside a function, it's local to that function. If you create a variable with var outside functions, it will be a global variable.

If you are declaring a global variable and set a value it won't have any practical value, but as mentioned, it's best practice. If however you want to declare a variable without a value, you will need "var".

javascript variable

do we have to declare js variable like this:
var x=5;
or just simple like this
x=5;
what is the differences?... will it effect the functionality of the variable?...

variable_name = 5 would always put the variable_name into the global object (which is window in a browser)
If you are in the global context (= not a function context) the two statements are basically the same. But if you are in a function context, var makes sure that this variable is only declared within the current context.
So for instance:
function foobar() {
bar = 55;
}
foobar();
window.bar === 55 // true
Better:
function foobar() {
var bar = 55;
}
window.bar === 55 // false
Conclusion: always use var within the context of a function. This avoids clobbering / overriding the global object with variables.

The var keyword limits the scope of the variable to the current function. Leaving it off makes a global. Globals are bad and should be avoided as they are a key source of race conditions and scripts interfering with each other.

x=5 means this variable (after execution) will become global and can be accessed by all other parts in your code(and also by other javascripts).
while var x=5 will be familiar only to the block which it was declared on, (and it's descendants)
Note that although x=5 will be global, it will become global only when this line is execute!
So if you place this inside a function, it will become global only after that function is called (for the first time).

It makes no difference - with the second one (x=5;) the variable is automatically declared if it does not yet exist.
I always use the first option however.

Is using 'var' to declare variables optional? [duplicate]

This question already has answers here:
What is the purpose of the var keyword and when should I use it (or omit it)?
(19 answers)
Closed 7 years ago.
Is "var" optional?
myObj = 1;
same as ?
var myObj = 1;
I found they both work from my test, I assume var is optional. Is that right?

They mean different things.
If you use var the variable is declared within the scope you are in (e.g. of the function). If you don't use var, the variable bubbles up through the layers of scope until it encounters a variable by the given name or the global object (window, if you are doing it in the browser), where it then attaches. It is then very similar to a global variable. However, it can still be deleted with delete (most likely by someone else's code who also failed to use var). If you use var in the global scope, the variable is truly global and cannot be deleted.
This is, in my opinion, one of the most dangerous issues with javascript, and should be deprecated, or at least raise warnings over warnings. The reason is, it's easy to forget var and have by accident a common variable name bound to the global object. This produces weird and difficult to debug behavior.

This is one of the tricky parts of Javascript, but also one of its core features. A variable declared with var "begins its life" right where you declare it. If you leave out the var, it's like you're talking about a variable that you have used before.
var foo = 'first time use';
foo = 'second time use';
With regards to scope, it is not true that variables automatically become global. Rather, Javascript will traverse up the scope chain to see if you have used the variable before. If it finds an instance of a variable of the same name used before, it'll use that and whatever scope it was declared in. If it doesn't encounter the variable anywhere it'll eventually hit the global object (window in a browser) and will attach the variable to it.
var foo = "I'm global";
var bar = "So am I";
function () {
var foo = "I'm local, the previous 'foo' didn't notice a thing";
var baz = "I'm local, too";
function () {
var foo = "I'm even more local, all three 'foos' have different values";
baz = "I just changed 'baz' one scope higher, but it's still not global";
bar = "I just changed the global 'bar' variable";
xyz = "I just created a new global variable";
}
}
This behavior is really powerful when used with nested functions and callbacks. Learning about what functions are and how scope works is the most important thing in Javascript.

Nope, they are not equivalent.
With myObj = 1; you are using a global variable.
The latter declaration create a variable local to the scope you are using.
Try the following code to understand the differences:
external = 5;
function firsttry() {
var external = 6;
alert("first Try: " + external);
}
function secondtry() {
external = 7;
alert("second Try: " + external);
}
alert(external); // Prints 5
firsttry(); // Prints 6
alert(external); // Prints 5
secondtry(); // Prints 7
alert(external); // Prints 7
The second function alters the value of the global variable "external", but the first function doesn't.

There's a bit more to it than just local vs global. Global variables created with var are different than those created without. Consider this:
var foo = 1; // declared properly
bar = 2; // implied global
window.baz = 3; // global via window object
Based on the answers so far, these global variables, foo, bar, and baz are all equivalent. This is not the case. Global variables made with var are (correctly) assigned the internal [[DontDelete]] property, such that they cannot be deleted.
delete foo; // false
delete bar; // true
delete baz; // true
foo; // 1
bar; // ReferenceError
baz; // ReferenceError
This is why you should always use var, even for global variables.

There's so much confusion around this subject, and none of the existing answers cover everything clearly and directly. Here are some examples with comments inline.
//this is a declaration
var foo;
//this is an assignment
bar = 3;
//this is a declaration and an assignment
var dual = 5;
A declaration sets a DontDelete flag. An assignment does not.
A declaration ties that variable to the current scope.
A variable assigned but not declared will look for a scope to attach itself to. That means it will traverse up the food-chain of scope until a variable with the same name is found. If none is found, it will be attached to the top-level scope (which is commonly referred to as global).
function example(){
//is a member of the scope defined by the function example
var foo;
//this function is also part of the scope of the function example
var bar = function(){
foo = 12; // traverses scope and assigns example.foo to 12
}
}
function something_different(){
foo = 15; // traverses scope and assigns global.foo to 15
}
For a very clear description of what is happening, this analysis of the delete function covers variable instantiation and assignment extensively.

var is optional. var puts a variable in local scope. If a variable is defined without var, it is in global scope and not deletable.
edit
I thought that the non-deletable part was true at some point in time with a certain environment. I must have dreamed it.

Check out this Fiddle: http://jsfiddle.net/GWr6Z/2/
function doMe(){
a = "123"; // will be global
var b = "321"; // local to doMe
alert("a:"+a+" -- b:"+b);
b = "something else"; // still local (not global)
alert("a:"+a+" -- b:"+b);
};
doMe()
alert("a:"+a+" -- b:"+b); // `b` will not be defined, check console.log

They are not the same.
Undeclared variable (without var) are treated as properties of the global object. (Usually the window object, unless you're in a with block)
Variables declared with var are normal local variables, and are not visible outside the function they're declared in. (Note that Javascript does not have block scope)
Update: ECMAScript 2015
let was introduced in ECMAScript 2015 to have block scope.

The var keyword in Javascript is there for a purpose.
If you declare a variable without the var keyword, like this:
myVar = 100;
It becomes a global variable that can be accessed from any part of your script. If you did not do it intentionally or are not aware of it, it can cause you pain if you re-use the variable name at another place in your javascript.
If you declare the variable with the var keyword, like this:
var myVar = 100;
It is local to the scope ({] - braces, function, file, depending on where you placed it).
This a safer way to treat variables. So unless you are doing it on purpose try to declare variable with the var keyword and not without.

Consider this question asked at StackOverflow today:
Simple Javascript question
A good test and a practical example is what happens in the above scenario...
The developer used the name of the JavaScript function in one of his variables.
What's the problem with the code?
The code only works the first time the user clicks the button.
What's the solution?
Add the var keyword before the variable name.

Var doesn't let you, the programmer, declare a variable because Javascript doesn't have variables. Javascript has objects. Var declares a name to an undefined object, explicitly. Assignment assigns a name as a handle to an object that has been given a value.
Using var tells the Javacript interpreter two things:
not to use delegation reverse traversal look up value for the name, instead use this one
not to delete the name
Omission of var tells the Javacript interpreter to use the first-found previous instance of an object with the same name.
Var as a keyword arose from a poor decision by the language designer much in the same way that Javascript as a name arose from a poor decision.
ps. Study the code examples above.

Everything about scope aside, they can be used differently.
console.out(var myObj=1);
//SyntaxError: Unexpected token var
console.out(myObj=1);
//1
Something something statement vs expression

No, it is not "required", but it might as well be as it can cause major issues down the line if you don't. Not defining a variable with var put that variable inside the scope of the part of the code it's in. If you don't then it isn't contained in that scope and can overwrite previously defined variables with the same name that are outside the scope of the function you are in.

I just found the answer from a forum referred by one of my colleague. If you declare a variable outside a function, it's always global. No matter if you use var keyword or not. But, if you declare the variable inside a function, it has a big difference. Inside a function, if you declare the variable using var keyword, it will be local, but if you declare the variable without var keyword, it will be global. It can overwrite your previously declared variables. - See more at: http://forum.webdeveloperszone.com/question/what-is-the-difference-between-using-var-keyword-or-not-using-var-during-variable-declaration/#sthash.xNnLrwc3.dpuf