I am learning about For Looops and I find that the following two pieces of code work just the same.
Code 1:
for (let i=0 ; i<bobsFollowers.length; i++){
for ( let j=0; j<tinasFollowers.length; j++){
if (bobsFollowers[i] === tinasFollowers[j]){
mutualFollowers.push(tinasFollowers[i]);
}
}
}
console.log(mutualFollowers);
Code 2
for (i=0 ; i<bobsFollowers.length; i++){
for ( j=0; j<tinasFollowers.length; j++){
if (bobsFollowers[i] === tinasFollowers[j]){
mutualFollowers.push(tinasFollowers[i]);
}
}
}
console.log(mutualFollowers);
You can globally access the variable if you do not include let keyword
for (let i = 0; i < 3; i++) {
console.log(i);
}
for (j = 0; j < 3; j++) {
console.log(j);
}
console.log('j ', j); // accesible
console.log('i ', i); // inaccesible
Yes, if you do not specifically use the let keyword in a for, for...of or for...in loops etc, they will work the same as when you use the let keyword.
However, when you don't explicitly declare using let keyword, the variable will be declared as a var. let and const are block level variables, but you also need to understand that 'functions are not the only blocks, any { } is basically a block'. i.e. Once declared a 'var' will become a function scoped variable and in case it is not inside a function, and declared outside, then it becomes globally available, which is not such a good thing considering you might have multiple for loops using a variable named 'i' and if you don't use let, it will just keep incrementing the old 'i'. Please see the below two examples to understand what I mean by the above sentence:
const test = function(){
let let1 = 1;
var var1 = 2;
{
let let1 = 9; //This is a block scoped variable limited to this { } block
var var1 = 3; //re-initialization still overrides the original var 1 variable's value because once declared, the var variable is function scoped
console.log(let1); //9 - Block Scoped
console.log(var1); //3 - Function Scoped
}
console.log(let1); //1 - Function Scoped
console.log(var1); //3 - Still Function Scoped, always function scoped
}
test();
What I meant by globally incrementing value of 'i':
for(i=0; i< 2; i++){
console.log('first loop', i)
}
for(i;i< 5; i++){//I am not initializing i to 0, but I'm just trying to show that it is a global variable now.
console.log('second loop', i)
}
for(i; i< 7; i++){ //I am not initializing i to 0, but I'm just trying to show that it is a global variable now.
console.log('third loop', i)
}
Secondly, in ES5, Strict Mode was introduced, which basically is a way you can opt out of writing 'sloppy mode' JS code, and you can use it by specifying 'use strict'; globally or at a function level. And strict mode is used in most companies and developers in their professional code bases.
The point I want to convey with this is that 'Strict Mode' does not allow you to use for loops without declaring the variable, you have to explicitely specify the 'var' or 'let' keywords. Therefore it is always a good practice to declare your variables. You can read more about strict mode here: Strict Mode
Lastly, in the current post ES6 times, using the var keyword to declare variable is not considered as a good practice due to something called as Hoisting, which basically means that before the a var x; is declared, if you try to use it, you still can, however it's value will be 'undefined'. But in case of let you cannot use a variable before you declare it. You can read more about it Here : Hoisting
Also, in case of let if you try to access a variable before it's initialized, it throws you a Reference Error. This is because of a concept called as a 'Temporal Dead Zone'. You can read more about it here: TDZ
'let' is all about lexical scope, by default variables and objects in javascript have a global scope. As other have pointed out, using var will make the scope of variable global so anything can access and modify it.
Another use case for let would be in closures.
This example is from CS50's react native lecture 0 & 1.
Here makeFunctionArray returns an array of functions which print the value of i.
function makeFunctionArray() {
const array = [];
for(var i=0; i<5; i++) {
array.push(function () { console.log(i) });
}
return array;
}
const functionArray = makeFunctionArray();
functionArray[0]();
Now, what do you expect functionArray[0]() to print?
0, right? because we're invoking the function at index zero and it should console log 0.
But it doesn't print 0, instead it prints 5.
This is because 'i' has a global scope and will have a value of 5 when loop terminates.
the function(closure) we're returning from the makeArray function will still have access to 'i' and the value 5 gets wrapped up in it while being returned. so every functionArray[index]() will print 5.
This can be avoided with 'let' if 'i' is a let, its scope will only be the 'for' loop.
At the end of this lecture, this scenario is introduced and answered in next lecture
Yes, you have to. Or the old var
As other responses mention you will populate this variable in a broader scope.
Imagine this situation, You need to create a function that print the first 10 numbers:
function printNumbers(){
for (i=0; i<10; i++) {
console.log(i)
}
}
That's cool. Now you need to invoke this function 12 times, easy right?
function printNumbers(){
for (i=0; i<10; i++) {
console.log(i)
}
}
for (i=0; i<12; i++) {
printNumbers()
}
Well, if you execute this in your google chrome console (don't). You will fry your browsers. Since i will never reach 12 and you will be in an infinite loop.
You can test this safely by changing 12 by 5, you will see that the function only runs 1 time and not 5.
So it is not for your code (only), it is for who is going to use your code in the future. You are leaving a potential big fail in your code. Imagine it is in a library.
You have a good explanation already in StackOverflow about scoping and let here: What's the difference between using "let" and "var"?
Also about scope in general here: What is the scope of variables in JavaScript?
The scope of let is restricted to that block alone. but if you do not specify let keyword it is scoped to the global environment(the global scope of the browser is window object)
{
let a = 1;
b = 2;
}
console.log(a) //error
console.log(b) //2
Related
Question title seems straight forward but that's not what I am asking. Please try following examples
for(let i=0; i<5; i++){
//console.log(i);
let i=5
console.log(i);
}
This example works and my question is how is it possible to declare another variable with same identifier inside for loop.
I am really confused, what is going on??
For the semantics of let in for loops, see Explanation of `let` and block scoping with for loops.
How is it possible to declare another variable with same identifier inside for loop?
It's because your for loop has a block statement as its body, and in there you can declare your own block-scoped variables, shadowing the ones from parent scopes.
In the following example you set the value of i to 5 so the loop ends coz that's the condition for the loop i<5. So condition is false for the loop to run again
for(let i=0; i<5; i++){
//console.log(i);
i=5
console.log(i);
}
And in your firs example, loop continues coz let i =5 is considered a separate variable in the for loop scope
I think below code will remove all your doubt It is all about scope, You can not re-define same variable in same scope but you can define in nested scope
Same thing happening in your code for has own scope and inside for means
for { //inside this } is nested scope of `for`.
{
let a = "a";
console.log(a);
{
let a = "b";
console.log(a);
}
console.log(a);
}
It seems like a lot of people are talking around the real point of confusion here...
Ok, so when you say
// scope A
for(let i = 0; i<5; i++) {
//scope B
}
we know i is not declared in Scope A. (That's one of the big problems let solves.) So it seems it must be declared in Scope B. But what about this case?
// scope A
for(let i = 0; i<5; i++) console.log(i);
The for loop still has to have its own scope, in order to keep i from being in Scope A. And we can demonstrate this.
let i = 37;
for(let i = 0; i < 5; i++) console.log(i);
console.log(i); // prints 37
Now if the for statement already has a scope even though it has no braces, then what happens when we replace console.log(i) with a block statement? Well, that block statement gets its own scope. So really we have
// Scope A
for( //scope B is actually here
let i = 0; i < 5; i++) {
// Scope C
}
Having this two options:
Option A:
var index;
for (index in myObject) {
// Do something
}
Option B:
for (var index in myObject) {
// Do something
}
I don't know if in the option B the variable index it's being redeclared every time of the loop or just once.
Those two snippets of code do exactly the same thing (and that's the case in most language such as C, C++ and C# amongst others). If the variable was redeclared at every iteration, then following your logic, it would also be re-initialized, and would constantly loop over the same object. Your loop would be infinite.
On a side-note, in JavaScript, all variable declarations get pushed to the function scope; this means that you can declare variables anywhere within a function, even within nested loops, and they will only be declared once.
Link to the var documentation
Relevant SO question
Other relevant SO answer
Edit courtesy of #torazaburo:
If you want to declare a variable with a local scope (as in, a variable that will only be defined in the current block such as a for, while or if, you can use the let statement:
let var1 = 123;
It also allows you to override variables with the same name but declared in a higher scope, such as in this example from the docs:
function letTest() {
let x = 1;
if (true) {
let x = 2; // different variable
console.log(x); // 2
}
console.log(x); // 1
}
See the full documentation (and examples) here.
The preferred approach in 2016 is to declare the variable in the loop head, using let:
for (let i = 0; i < max; i++) { }
^^^
There is probably minimal performance difference between this and other approaches, but there are major advantages in terms of the robustness and clarity of your code.
First, with let, i is local to the for construct, so it can't "leak out", or modify some other i in an outer scope.
Second, and perhaps more importantly, a new version of i is created for each iteration of the loop. In technical terms, "you get a fresh binding for each iteration if you let-declare a variable" (see this excellent article). This solves the age-old problem of closures created inside the for-loop using the final value of i. We can now just write
for (let i = 0; i < 10; i++) {
setTimeout(() => alert(i), i * 1000);
}
instead of having to do some clumsy work-around such as
for (var i = 0; i < 10; i++) {
(function(i) {
setTimeout(() => alert(i), i * 1000);
}(i));
}
which has been the topic of countless questions here on SO, and which many of you have wasted far too many brain cycles learning.
This question already has answers here:
What is the scope of variables in JavaScript?
(27 answers)
Closed 6 years ago.
I thought declaring something with "var" made it local to its scope. Why, then, does this code seem to have not two distinct variables named "len", but just one?
for (var i=0, len=3; i<len; i++) {
for (var j=0, len=1; j<len; j++) {
console.log('i=' + i + ', j=' + j + ', len=' + len);
}
}
In the output, instead of three lines, I get only one. Why?
(True on Chrome and Safari, haven't checked others.)
Because variables declared with varare bound to the function scope, not the block scope like in many other languages. For example, this works just fine:
if (true) {
var thing = 'something';
}
console.log(thing); // 'something'
However, this will not work because the scope is defined by the function.
function a() {
var thing = 'something';
}
function b() {
console.log(thing); // ReferenceError
}
a();
b();
I thought declaring something with "var" made it local to its scope.
It does: Function scope (or global scope if you use it globally). Unlike variable declarations in many similar languages, var doesn't have block-scope. So your loop is terminating after outputting just one line because you're setting len to 1 in the initializer of the inner loop, and that's the same len as the one being used by the outer loop.
That and some other aspects of var were sufficiently troublesome that it's been fixed: In ES2015 (the latest JavaScript specification), the new let has block scope (and there's some special hand-waving around for loops), so your example would work with let:
// NOTE: Only works on browsers supporting `let`!
"use strict"; // (Some browsers don't support let outside of strict mode yet)
for (let i = 0, len = 3; i < len; i++) {
for (let j = 0, len = 1; j < len; j++) {
console.log('i=' + i + ', j=' + j + ', len=' + len);
}
}
But I wouldn't recommend using two nested lens like that. Even though JavaScript won't get confused, there's no guarantee the programmer coming after you won't.
let is one of the several really good things about ES2015. In all but a very few edge cases, let is the new var. Of course, until all of your target browsers have updated their JavaScript engines, you have to either stick with var or transpile.
You are right to think that declaring something as var makes it local to the scope. That is why when you do the comparison in the inner for loop, the value of len that is used is the one that was declared in the inner loop. Hence only one line of output.
This question already has answers here:
Why the for loop counter doesn't get destroyed after exiting the loop in javascript?
(3 answers)
Closed 9 years ago.
I ran into this little gem 5 minutes ago. I have been playing with JavaScript for a long while now and, since I follow best practice, I've never met such case, nor understand why it works while I supposed it shouldn't :
for (var i=0; i<10; i++){
// ... something
}
console.log("i=", i);
will output 10
How is i available outside the for block? I always thought the declaration part was to have a local variable only available within that block.
I always thought the declaration part was to have a local variable only available within that block.
Nope, not in JavaScript.
JavaScript loops (and most blocks in general) have no block scoping (until then next version rolls out with let.)
There are only two places where JavaScript does block scope at the moment and that's with clauses (you shouldn't use those anyway) and catch clauses.
Instead, JavaScript relies mostly on function scoping - variables declared in a function are local to that function.
In this case the declaration of i is outside the code block. In any case, Javascript doesn't have a block-level scope. Variables are either global, or within the scope of a function.
Because that's equivalent to:
var i=0;
while (i<10){
// ... something
i++;
}
In fact, loops do not even create their own scope at all:
var x = 0;
while (x < 10) {
x++;
var i = 5;
}
i; // 5
i will be available below where you have defined it, unless you close off the scope, like:
(function(){
for(var i=0; i<10; i++){
}
})();
console.log(i); // undefined
This should not matter though, because as long as you are using other loops below you can use the same increment variable (with the exception of nested loops), which will overwrite the other one. A problem could arise when you have a loop where you forget to use the keyword var. Always use var would be my recommendation. A closure is not usually necessary.
In a nested loop you can use the same increment variables again like:
for(var i=0; i<10; i++){
for(var n=2; n<44; n+=2){
}
}
// feel free to use `i` and `n` again
Or loops like this:
var ar1 = ['a', 'b', 'c'];
for(var i=0,l=ar1.length,n=0; i<l; i++,n+=2){
}
// feel free to use `i`, `l`, and `n` again
Personally, I find it a best practice to reserve vars i, n, c, and q for counters and l for length. Then I don't use them elsewhere, besides in loops.
In javascript, there's no "block scope", but "function scope". It means that, as soon as your variables are defined inside a function, they'll remain alive from the point that they're declared until the end of that function, whether inside or out side a block.
Here is the test case for js variable scope (from Secrets of the Javascript Ninja, sections 3.2.1 Scoping and functions)
Test case:
function outer(){
var a = 1;
function inner(){ /* does nothing */ }
var b = 2;
if (a == 1) {
var c = 3;
}
}
outer();
Result:
Perhaps I'm not aware of how for loop index variables get scoped, but I was very surprised when one of my loops didn't complete, seemingly because a function called from within a loop contained an i for its for loop index as well.
Here's a little script I put together to demonstrate this behavior:
var loopOne = function(test) {
for(i = 0; i < test.length; i++)
console.log(getMask(test));
};
var getMask = function(pass) {
var s = "";
for (i = 0; i < pass.length; i++) {
s = s + "*";
}
return s;
};
loopOne('hello');
If I run this in Chrome and look at the console log, I should see ***** five times. However, I only see it once. Upon further inspection, if I type i in the Chrome javascript console, it will output 6 ( = 'hello'.length + 1). This makes me think that i has become a part of the global scope and is not limited to the scope of the for loop for which it was needed.
Is this correct? If so, what's a better practice for defining the index variable of a for loop in javascript?
In Javascript, variables are scoped with the var keyword. When declaring variables with var, the variable is scoped to the current function. When assigning to a variable without using the var keyword, it is assumed you're talking about an already defined variable in the same or a higher scope. If none is found, the variable is created in the highest scope.
Bottom line: declare all your variables using var.
You should declare your loop index variable with let:
for (let i = 0; i < test.length; i++) ...
That will declare and set the proper scope for the variable.
When you declare your variables with var, you scope them to the current execution context.
When you don't, they become properties of the global object (window in a browser).