By nickf


2009-04-15 06:06:20 8 Comments

var funcs = [];
for (var i = 0; i < 3; i++) {      // let's create 3 functions
  funcs[i] = function() {          // and store them in funcs
    console.log("My value: " + i); // each should log its value.
  };
}
for (var j = 0; j < 3; j++) {
  funcs[j]();                      // and now let's run each one to see
}

It outputs this:

My value: 3
My value: 3
My value: 3

Whereas I'd like it to output:

My value: 0
My value: 1
My value: 2


The same problem occurs when the delay in running the function is caused by using event listeners:

var buttons = document.getElementsByTagName("button");
for (var i = 0; i < buttons.length; i++) {          // let's create 3 functions
  buttons[i].addEventListener("click", function() { // as event listeners
    console.log("My value: " + i);                  // each should log its value.
  });
}
<button>0</button><br>
<button>1</button><br>
<button>2</button>

… or asynchronous code, e.g. using Promises:

// Some async wait function
const wait = (ms) => new Promise((resolve, reject) => setTimeout(resolve, ms));

for(var i = 0; i < 3; i++){
  wait(i * 100).then(() => console.log(i)); // Log `i` as soon as each promise resolves.
}

What’s the solution to this basic problem?

30 comments

@Nouman Dilshad 2018-09-05 13:00:37

Just change the var keyword to let.

var is function scoped.

let is block scoped.

When you start you code the for loop will iterate and assign the value of i to 3, which will remain 3 throughout your code. I suggest you to read more about scopes in node (let,var,const and others)

funcs = [];
for (let i = 0; i < 3; i++) {      // let's create 3 functions
  funcs[i] =async function() {          // and store them in funcs
    await console.log("My value: " + i); // each should log its value.
  };
}
for (var j = 0; j < 3; j++) {
  funcs[j]();                      // and now let's run each one to see
}

@user1559625 2018-08-01 09:32:05

This proves how ugly javascript is with regard to how 'closure' and 'non-closure' works.

In the case of:

var funcs = [];

for (var i = 0; i < 3; i++) {      // let's create 3 functions
  funcs[i] = function() {          // and store them in funcs
    console.log("My value: " + i); // each should log its value.
  };
}

funcs[i] is a global function, and 'console.log("My value: " + i);' is printing global variable i

In the case of

var funcs = [];

function createfunc(i) {
    return function() { console.log("My value: " + i); };
}

for (var i = 0; i < 3; i++) {
    funcs[i] = createfunc(i);
}

because of this twisted closure design of javascript, 'console.log("My value: " + i);' is printing the i from outer function 'createfunc(i)'

all because javascript can not design something decent like the 'static' variable inside a function like what C programming language is doing!

@ashish yadav 2018-07-13 08:02:09

var funcs = [];
for (var i = 0; i < 3; i++) {      // let's create 3 functions
  funcs[i] = function(param) {          // and store them in funcs
    console.log("My value: " + param); // each should log its value.
  };
}
for (var j = 0; j < 3; j++) {
  funcs[j](j);                      // and now let's run each one to see with j
}

@harto 2009-04-15 06:18:17

Well, the problem is that the variable i, within each of your anonymous functions, is bound to the same variable outside of the function.

Classic solution: Closures

What you want to do is bind the variable within each function to a separate, unchanging value outside of the function:

var funcs = [];

function createfunc(i) {
    return function() { console.log("My value: " + i); };
}

for (var i = 0; i < 3; i++) {
    funcs[i] = createfunc(i);
}

for (var j = 0; j < 3; j++) {
    funcs[j]();                        // and now let's run each one to see
}

Since there is no block scope in JavaScript - only function scope - by wrapping the function creation in a new function, you ensure that the value of "i" remains as you intended.


2015 Solution: forEach

With the relatively widespread availability of the Array.prototype.forEach function (in 2015), it's worth noting that in those situations involving iteration primarily over an array of values, .forEach() provides a clean, natural way to get a distinct closure for every iteration. That is, assuming you've got some sort of array containing values (DOM references, objects, whatever), and the problem arises of setting up callbacks specific to each element, you can do this:

var someArray = [ /* whatever */ ];
// ...
someArray.forEach(function(arrayElement) {
  // ... code code code for this one element
  someAsynchronousFunction(arrayElement, function() {
    arrayElement.doSomething();
  });
});

The idea is that each invocation of the callback function used with the .forEach loop will be its own closure. The parameter passed in to that handler is the array element specific to that particular step of the iteration. If it's used in an asynchronous callback, it won't collide with any of the other callbacks established at other steps of the iteration.

If you happen to be working in jQuery, the $.each() function gives you a similar capability.


ES6 solution: let

ECMAScript 6 (ES6), the newest version of JavaScript, is now starting to be implemented in many evergreen browsers and backend systems. There are also transpilers like Babel that will convert ES6 to ES5 to allow usage of new features on older systems.

ES6 introduces new let and const keywords that are scoped differently than var-based variables. For example, in a loop with a let-based index, each iteration through the loop will have a new value of i where each value is scoped inside the loop, so your code would work as you expect. There are many resources, but I'd recommend 2ality's block-scoping post as a great source of information.

for (let i = 0; i < 3; i++) {
    funcs[i] = function() {
        console.log("My value: " + i);
    };
}

Beware, though, that IE9-IE11 and Edge prior to Edge 14 support let but get the above wrong (they don't create a new i each time, so all the functions above would log 3 like they would if we used var). Edge 14 finally gets it right.

@PCA 2013-08-17 13:08:08

That was a good explanation, i have been reading through closures, but not clearly understood how the i value is not unique as 1,2,3. But after reading your post, i understood that javascript does not have block scope and only have function scope. Thanks for the simple answer.

@アレックス 2014-03-28 03:45:40

isn't function createfunc(i) { return function() { console.log("My value: " + i); }; } still closure because it uses the variable i?

@cookie monster 2014-04-04 19:30:21

@Alex: Yes. All JavaScript functions create closures. The only question is the state of the closed-over variable that the function references. The solution creates a new variable scope upon invocation of createfunc that has both the i value at the time of invocation and a function that references that i. So each returned function (closure) is in its own scope closing over its own i.

@Wladimir Palant 2014-06-20 12:21:23

Unfortunately, this answer is outdated and nobody will see the correct answer at the bottom - using Function.bind() is definitely preferable by now, see stackoverflow.com/a/19323214/785541.

@cookie monster 2014-07-12 02:35:51

@Wladimir: Your suggestion that .bind() is "the correct answer" isn't right. They each have their own place. With .bind() you can't bind arguments without binding the this value. Also you get a copy of the i argument without the ability to mutate it between calls, which sometimes is needed. So they're quite different constructs, not to mention that .bind() implementations have been historically slow. Sure in the simple example either would work, but closures are an important concept to understand, and that's what the question was about.

@greenoldman 2014-11-21 07:14:51

@LoïcFaure-Lacroix, how let helps in capturing the values, and not references?

@Christian Landgren 2015-02-07 10:23:16

Please stop using these for-return function hacks, use [].forEach or [].map instead because they avoid reusing the same scope variables.

@user1106925 2015-06-29 16:31:47

@ChristianLandgren: That's only useful if you're iterating an Array. These techniques aren't "hacks". They're essential knowledge.

@Nirmit Srivastava 2015-08-28 11:07:15

wow!!! this was answered in 2009 and it still works in 2015.

@Carsten Farving 2015-09-22 06:09:00

@ChristianLandgren For instance in asynchronous callbacks, you might need to use these techniques because you don't have an array to iterate over. NirmitSrivastava Mostly what works in the early years of javascript is still compliant to whatever used today.

@Christian Landgren 2015-09-22 07:15:55

@CarstenFarving When dealing with asynchronous callbacks it is essential that you keep a reference to a scoped variable since the parent value may be changed between the time you create a request until you get the response. If you don't have an array, just create one with [1,2,3].forEach((i) => doSomething(i))

@arnabkaycee 2015-11-26 10:12:08

This is a basic problem that 'var' variables face, i.e. they are function scoped instead of block scoped. 'var' variables undergo hoisting. This problem is overcome by 'let' statement introduced in ES6. See. developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…

@JSG 2016-04-18 20:16:01

The "let" method seems to work great for me being that it does work exactly how i intended the script to work before i stumbled upon closure issues. I needed to create the function on the go restoring previous functionality to onclick behavior. In a parent function vars are set and computed and passed through.

@Costa 2016-12-07 17:03:28

This isn't actually that complicated. The relevant question is from @アレックス: "Isn't function createfunc(i) { return function() { console.log("My value: " + i); }; } still closure because it uses the variable i?" The answer is, Yes, of course! Now you're getting it. When a function is declared is closes over the scope in which it was declared. createFunc(i) is a whole new closure, one level deeper. Remember how functions use the nearest variable reference? What's a bit confusing is that we used i as the argument for createFunc instead of n or some other random counter.

@nnnnnn 2016-12-12 03:01:02

Note that using let within the for loop's () doesn't work properly in IE (one workaround: add a let variable inside the {} block of the for, which does work properly in IE).

@Somnium 2017-02-13 12:11:33

Using let is best approach in new browsers, however it is more correct to add let inside for body, as @nnnnnn suggests, because for loop header has its own scope (however it works in non-IE browsers for some reason when it shouldn't).

@nnnnnn 2017-02-13 14:10:38

@Somnium - it's IE that implements it incorrectly. The other browsers do what the spec says.

@Pawel 2017-02-27 16:36:57

@WladimirPalant the answer you link to is a worse solution than this one. "bind" is good for setTimeout and callbacks but not for using it in a loop

@Wladimir Palant 2017-02-27 17:23:44

@Pawel: This question is about creating instances of a particular function bound to specific parameters - this is exactly what Function.bind() is good for. The loop merely demonstrates the principle, it's simply about binding variables that can change after the fact. In an actual loop you could use a let statement by now of course but support for it is only now becoming sufficient to be used unfortunately.

@Pawel 2017-02-28 15:01:27

@WladimirPalant bind primary function is to bind the context not attributes. Attributes are in addition to the context so bind in this case is an overkill. Compared to returning a function bind is 3 times slower in Chrome and 10 times slower in Firefox and Edge.

@arun tiwari 2018-02-13 14:26:37

The answer is explained very well

@mbomb007 2018-03-23 21:17:58

IE11 actually supports let correctly. See caniuse.com/#search=let

@JLRishe 2018-04-04 20:20:28

@mbomb007 No it doesn't. Try running the following in IE, Firefox, and Chrome. You will get different results: (function () { for(let a = 0; a < 10; a += 1) { setTimeout(function() { console.log(a); }, 100); } })()

@mbomb007 2018-04-04 21:27:51

@JLRishe Someone should tell caniuse.com to add note #4 to IE11 then

@JLRishe 2018-04-05 07:22:44

@mbomb007 Seems like it's a little different from #4 but I have submitted a PR to attempt to correct their data.

@stemon 2018-05-25 00:18:33

Why not simply call each function inside the first (and only) loop just after they were created, such as:

 var funcs = [];
    for (var i = 0; i < 3; i++) {
    // let's create 3 functions
    funcs[i] = function() {
    // and store them in funcs
    console.log("My value: " + i); // each should log its value.
    };
    funcs[i]();// and now let's run each one to see
    }

@nickf 2018-05-31 09:22:48

Because it was just an example of the problem.

@Ben McCormick 2013-05-21 03:04:48

With ES6 now widely supported, the best answer to this question has changed. ES6 provides the let and const keywords for this exact circumstance. Instead of messing around with closures, we can just use let to set a loop scope variable like this:

var funcs = [];
for (let i = 0; i < 3; i++) {          
    funcs[i] = function() {            
      console.log("My value: " + i); 
    };
}

val will then point to an object that is specific to that particular turn of the loop, and will return the correct value without the additional closure notation. This obviously significantly simplifies this problem.

const is similar to let with the additional restriction that the variable name can't be rebound to a new reference after initial assignment.

Browser support is now here for those targeting the latest versions of browsers. const/let are currently supported in the latest Firefox, Safari, Edge and Chrome. It also is supported in Node, and you can use it anywhere by taking advantage of build tools like Babel. You can see a working example here: http://jsfiddle.net/ben336/rbU4t/2/

Docs here:

Beware, though, that IE9-IE11 and Edge prior to Edge 14 support let but get the above wrong (they don't create a new i each time, so all the functions above would log 3 like they would if we used var). Edge 14 finally gets it right.

@MattC 2016-02-23 17:47:32

Unfortunately, 'let' is still not fully supported, especially in mobile. developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…

@Dan Pantry 2016-06-22 10:18:14

As of June '16, let is supported in all major browser versions except iOS Safari, Opera Mini and Safari 9. Evergreen browsers support it. Babel will transpile it correctly to keep the expected behaviour without high compliancy mode switched on.

@Ben McCormick 2016-06-27 14:24:27

@DanPantry yeah about time for an update :) Updated to better reflect the current state of things, including adding a mention of const, doc links and better compatibility info.

@pixel 67 2018-03-19 15:56:00

Isn't this why we use babel to transpile our code so browsers that don't support ES6/7 can understand what's going on?

@Mark Manning 2018-03-28 20:15:11

Ok. I read through all of the answers. Even though there is a good explanation here - I just could not get this to work. So I went looking on the internet. The person at https://dzone.com/articles/why-does-javascript-loop-only-use-last-value had an answer which is not presented here. So I thought I'd post a short example. This made a lot more sense to me.

The long and short of it is that the LET command is nice but is only now being used. HOWEVER, the LET command is really just a TRY-CATCH combo. This works all the way back to IE3 (I believe). Using the TRY-CATCH combo - life is simple and good. Probably why the EMCScript people decided to use it. It also does not need a setTimeout() function. So no time is lost. Basically, you need one TRY-CATCH combo per FOR loop. Here is an example:

    for( var i in myArray ){
       try{ throw i }
       catch(ii){
// Do whatever it is you want to do with ii
          }
       }

If you have more than one FOR loop, you just put a TRY-CATCH combo for each one. Also, personally, I always use the double letter of whatever FOR variable I am using. So "ii" for "i" and so on. I am using this technique in a routine to send mouseover commands to a different routine.

@Eyal Segal 2018-03-14 06:19:14

Let's say you don't use es6; You can use IFFY function:

var funcs = [];
for (var i = 0; i < 13; i++) {      
funcs[i] = (function(x) {
console.log("My value: " + i)})(i);}

But it will be different.

@woojoo666 2015-04-10 09:57:07

Bit late to the party, but I was exploring this issue today and noticed that many of the answers don't completely address how Javascript treats scopes, which is essentially what this boils down to.

So as many others mentioned, the problem is that the inner function is referencing the same i variable. So why don't we just create a new local variable each iteration, and have the inner function reference that instead?

//overwrite console.log() so you can see the console output
console.log = function(msg) {document.body.innerHTML += '<p>' + msg + '</p>';};

var funcs = {};
for (var i = 0; i < 3; i++) {
    var ilocal = i; //create a new local variable
    funcs[i] = function() {
        console.log("My value: " + ilocal); //each should reference its own local variable
    };
}
for (var j = 0; j < 3; j++) {
    funcs[j]();
}

Just like before, where each inner function outputted the last value assigned to i, now each inner function just outputs the last value assigned to ilocal. But shouldn't each iteration have it's own ilocal?

Turns out, that's the issue. Each iteration is sharing the same scope, so every iteration after the first is just overwriting ilocal. From MDN:

Important: JavaScript does not have block scope. Variables introduced with a block are scoped to the containing function or script, and the effects of setting them persist beyond the block itself. In other words, block statements do not introduce a scope. Although "standalone" blocks are valid syntax, you do not want to use standalone blocks in JavaScript, because they don't do what you think they do, if you think they do anything like such blocks in C or Java.

Reiterated for emphasis:

JavaScript does not have block scope. Variables introduced with a block are scoped to the containing function or script

We can see this by checking ilocal before we declare it in each iteration:

//overwrite console.log() so you can see the console output
console.log = function(msg) {document.body.innerHTML += '<p>' + msg + '</p>';};

var funcs = {};
for (var i = 0; i < 3; i++) {
  console.log(ilocal);
  var ilocal = i;
}

This is exactly why this bug is so tricky. Even though you are redeclaring a variable, Javascript won't throw an error, and JSLint won't even throw a warning. This is also why the best way to solve this is to take advantage of closures, which is essentially the idea that in Javascript, inner functions have access to outer variables because inner scopes "enclose" outer scopes.

Closures

This also means that inner functions "hold onto" outer variables and keep them alive, even if the outer function returns. To utilize this, we create and call a wrapper function purely to make a new scope, declare ilocal in the new scope, and return an inner function that uses ilocal (more explanation below):

//overwrite console.log() so you can see the console output
console.log = function(msg) {document.body.innerHTML += '<p>' + msg + '</p>';};

var funcs = {};
for (var i = 0; i < 3; i++) {
    funcs[i] = (function() { //create a new scope using a wrapper function
        var ilocal = i; //capture i into a local var
        return function() { //return the inner function
            console.log("My value: " + ilocal);
        };
    })(); //remember to run the wrapper function
}
for (var j = 0; j < 3; j++) {
    funcs[j]();
}

Creating the inner function inside a wrapper function gives the inner function a private environment that only it can access, a "closure". Thus, every time we call the wrapper function we create a new inner function with it's own separate environment, ensuring that the ilocal variables don't collide and overwrite each other. A few minor optimizations gives the final answer that many other SO users gave:

//overwrite console.log() so you can see the console output
console.log = function(msg) {document.body.innerHTML += '<p>' + msg + '</p>';};

var funcs = {};
for (var i = 0; i < 3; i++) {
    funcs[i] = wrapper(i);
}
for (var j = 0; j < 3; j++) {
    funcs[j]();
}
//creates a separate environment for the inner function
function wrapper(ilocal) {
    return function() { //return the inner function
        console.log("My value: " + ilocal);
    };
}

Update

With ES6 now mainstream, we can now use the new let keyword to create block-scoped variables:

//overwrite console.log() so you can see the console output
console.log = function(msg) {document.body.innerHTML += '<p>' + msg + '</p>';};

var funcs = {};
for (let i = 0; i < 3; i++) { // use "let" to declare "i"
    funcs[i] = function() {
        console.log("My value: " + i); //each should reference its own local variable
    };
}
for (var j = 0; j < 3; j++) { // we can use "var" here without issue
    funcs[j]();
}

Look how easy it is now! For more information see this answer, which my info is based off of.

@CapturedTree 2017-10-26 21:45:21

I like how you explained the IIFE way as well. I was looking for that. Thank you.

@Tiny Giant 2017-12-27 03:12:06

There is now such a thing as block scoping in JavaScript using the let and const keywords. If this answer were to expand to include that, it would be much more globally useful in my opinion.

@woojoo666 2018-03-01 22:44:09

@TinyGiant sure thing, I added some info about let and linked a more complete explanation

@nutty about natty 2018-05-14 19:08:24

@woojoo666 Could your answer also work for calling two alternating URL's in a loop like so: i=0; while(i < 100) { setTimeout(function(){ window.open("https://www.bbc.com","_self") }, 3000); setTimeout(function(){ window.open("https://www.cnn.com","_self") }, 3000); i++ }? (could replace window.open() with getelementbyid......)

@woojoo666 2018-06-03 22:58:23

@nuttyaboutnatty sorry about such a late reply. It doesn't seem like the code in your example already works. You aren't using i in your timeout functions, so you don't need a closure

@woojoo666 2018-06-08 11:22:45

whoops, sorry, meant to say "it seems like the code in your example already works"

@Brooks DuBois 2018-02-25 03:15:21

While this question is old and answered, I have yet another fairly interesting solution:

var funcs = [];

for (var i = 0; i < 3; i++) {     
  funcs[i] = function() {          
    console.log("My value: " + i); 
 };
}

for (var i = 0; i < 3; i++) {
  funcs[i]();
}

The change is so small it's almost difficult to see what I did. I switched the second iterator from a j to an i. This somehow refreshes the state of i in time to give you the desired result. I did this by accident but it makes sense considering previous answers.

I wrote this up to point out this small, yet very important difference. Hope that helps to clear up some confusion for other learners like me.

Note: I am not sharing this because I think it's the right answer. This is a flakey solution that probably will break under certain circumstances. Actually, I'm quite amazed that it really works.

@nickf 2018-03-15 21:04:45

It's only working because in the second loop, you're overwriting the same i as is referenced in the function. Just consider that in this whole snippet, there is only one i variable. It is equivalent to: i = 0; funcs[0](); i = 1; funcs[1](); ..

@Brooks DuBois 2018-03-16 00:51:31

right, which makes sense considering the other answers about scoping, but still is sort of counterintuitive

@Aust 2013-10-11 16:41:56

Another way that hasn't been mentioned yet is the use of Function.prototype.bind

var funcs = {};
for (var i = 0; i < 3; i++) {
  funcs[i] = function(x) {
    console.log('My value: ' + x);
  }.bind(this, i);
}
for (var j = 0; j < 3; j++) {
  funcs[j]();
}

UPDATE

As pointed out by @squint and @mekdev, you get better performance by creating the function outside the loop first and then binding the results within the loop.

function log(x) {
  console.log('My value: ' + x);
}

var funcs = [];

for (var i = 0; i < 3; i++) {
  funcs[i] = log.bind(this, i);
}

for (var j = 0; j < 3; j++) {
  funcs[j]();
}

@Bjorn Tipling 2014-12-08 05:18:31

This is what I do these days too, I also like lo-dash/underscore's _.partial

@user1106925 2015-06-28 03:29:07

.bind() will be largely obsolete with ECMAScript 6 features. Besides, this actually creates two functions per iteration. First the anonymous, then the one generated by .bind(). Better use would be to create it outside the loop, then .bind() it inside.

@mekdev 2015-06-28 18:32:17

Doesn't this trigger JsHint - Don't make functions within a loop. ? I went down this path also but after running code quality tools its a no go..

@Aust 2015-06-29 16:23:27

@squint @mekdev - You both are correct. My initial example was written quickly to demonstrate how bind is used. I've added another example per your suggestions.

@user2290820 2015-09-11 12:14:20

I think instead of wasting computation over two O(n) loops, just do for (var i = 0; i < 3; i++) { log.call(this, i); }

@niry 2017-01-08 05:55:07

.bind() does what the accepted answer suggests PLUS fiddles with this.

@Bimal Das 2018-01-16 14:29:57

We will check , what actually happens when you declare var and let one by one.

Case1 : using var

<script>
   var funcs = [];
   for (var i = 0; i < 3; i++) {
     funcs[i] = function () {
        debugger;
        console.log("My value: " + i);
     };
   }
   console.log(funcs);
</script>

Now open your chrome console window by pressing F12 and refresh the page. Expend every 3 functions inside the array.You will see an property called [[Scopes]].Expand that one. You will see one array object called "Global",expand that one. You will find a property 'i' declared into the object which having value 3.

enter image description here

enter image description here

Conclusion:

  1. When you declare a variable using 'var' outside a function ,it becomes global variable(you can check by typing i or window.i in console window.It will return 3).
  2. The annominous function you declared will not call and check the value inside the function unless you invoke the functions.
  3. When you invoke the function , console.log("My value: " + i) takes the value from its Global object and display the result.

CASE2 : using let

Now replace the 'var' with 'let'

<script>
    var funcs = [];
    for (let i = 0; i < 3; i++) {
        funcs[i] = function () {
           debugger;
           console.log("My value: " + i);
        };
    }
    console.log(funcs);
</script>

Do the same thing, Go to the scopes . Now you will see two objects "Block" and "Global". Now expand Block object , you will see 'i' is defined there , and the strange thing is that , for every functions , the value if i is different (0 , 1, 2).

enter image description here

Conclusion:

When you declare variable using 'let' even outside the function but inside the loop , this variable will not be a Global variable , it will become a Block level variable which is only available for the same function only.That is the reason , we are getting value of i different for each function when we invoke the functions.

For more detail about how closer works , please go through the awesome video tutorial https://youtu.be/71AtaJpJHw0

@sidhuko 2018-01-13 13:17:57

This question really shows the history of JavaScript! Now we can avoid block scoping with arrow functions and handle loops directly from DOM nodes using Object methods.

const funcs = [1, 2, 3].map(i => () => console.log(i));
funcs.map(fn => fn())

const buttons = document.getElementsByTagName("button");
Object
  .keys(buttons)
  .map(i => buttons[i].addEventListener('click', () => console.log(i)));
<button>0</button><br>
<button>1</button><br>
<button>2</button>

@Kemal Dağ 2013-06-25 14:21:53

The most simple solution would be,

Instead of using:

var funcs = [];
for(var i =0; i<3; i++){
    funcs[i] = function(){
        alert(i);
    }
}

for(var j =0; j<3; j++){
    funcs[j]();
}

which alerts "2", for 3 times. This is because anonymous functions created in for loop, shares same closure, and in that closure, the value of i is the same. Use this to prevent shared closure:

var funcs = [];
for(var new_i =0; new_i<3; new_i++){
    (function(i){
        funcs[i] = function(){
            alert(i);
        }
    })(new_i);
}

for(var j =0; j<3; j++){
    funcs[j]();
}

The idea behind this is, encapsulating the entire body of the for loop with an IIFE (Immediately-Invoked Function Expression) and passing new_i as a parameter and capturing it as i. Since the anonymous function is executed immediately, the i value is different for each function defined inside the anonymous function.

This solution seems to fit any such problem since it will require minimal changes to the original code suffering from this issue. In fact, this is by design, it should not be an issue at all!

@DanMan 2013-07-26 11:18:39

Read something similar in a book once. I prefer this, too, since you don't have to touch your existing code (as much) and it becomes obvious why you did it, once you've learned the self-calling function pattern: to trap that variable in the newly created scope.

@Kemal Dağ 2013-07-26 12:20:44

@DanMan Thanks. Self calling anonymous functions are very good way to deal javascript's lack of block level variable scope.

@jherax 2015-10-27 04:29:35

Self-calling, or self-invoking is not the appropriate term for this technique, IIFE (Immediately-Invoked Function Expression) is more accurately. Ref: benalman.com/news/2010/11/…

@axelduch 2015-05-06 22:33:08

This is a problem often encountered with asynchronous code, the variable i is mutable and at the time at which the function call is made the code using i will be executed and i will have mutated to its last value, thus meaning all functions created within the loop will create a closure and i will be equal to 3 (the upper bound + 1 of the for loop.

A workaround to this, is to create a function that will hold the value of i for each iteration and force a copy i (as it is a primitive, think of it as a snapshot if it helps you).

@Vikash Singh 2017-01-20 10:02:32

Use closure structure, this would reduce your extra for loop. You can do it in a single for loop:

var funcs = [];
for (var i = 0; i < 3; i++) {     
  (funcs[i] = function() {         
    console.log("My value: " + i); 
  })(i);
}

@jsbisht 2017-07-17 10:10:55

COUNTER BEING A PRIMITIVE

Let's define callback functions as follows:

// ****************************
// COUNTER BEING A PRIMITIVE
// ****************************
function test1() {
    for (var i=0; i<2; i++) {
        setTimeout(function() {
            console.log(i);
        });
    }
}
test1();
// 2
// 2

After timeout completes it will print 2 for both. This is because the callback function accesses the value based on the lexical scope, where it was function was defined.

To pass and preserve the value while callback was defined, we can create a closure, to preserve the value before the callback is invoked. This can be done as follows:

function test2() {
    function sendRequest(i) {
        setTimeout(function() {
            console.log(i);
        });
    }

    for (var i = 0; i < 2; i++) {
        sendRequest(i);
    }
}
test2();
// 1
// 2

Now what's special about this is "The primitives are passed by value and copied. Thus when the closure is defined, they keep the value from the previous loop."

COUNTER BEING AN OBJECT

Since closures have access to parent function variables via reference, this approach would differ from that for primitives.

// ****************************
// COUNTER BEING AN OBJECT
// ****************************
function test3() {
    var index = { i: 0 };
    for (index.i=0; index.i<2; index.i++) {
        setTimeout(function() {
            console.log('test3: ' + index.i);
        });
    }
}
test3();
// 2
// 2

So, even if a closure is created for the variable being passed as an object, the value of the loop index will not be preserved. This is to show that the values of an object are not copied whereas they are accessed via reference.

function test4() {
    var index = { i: 0 };
    function sendRequest(index, i) {
        setTimeout(function() {
            console.log('index: ' + index);
            console.log('i: ' + i);
            console.log(index[i]);
        });
    }

    for (index.i=0; index.i<2; index.i++) {
        sendRequest(index, index.i);
    }
}
test4();
// index: { i: 2}
// 0
// undefined

// index: { i: 2}
// 1
// undefined

@Buksy 2016-11-04 08:58:29

Your code doesn't work, because what it does is:

Create variable `funcs` and assign it an empty array;  
Loop from 0 up until it is less than 3 and assign it to variable `i`;
    Push to variable `funcs` next function:  
        // Only push (save), but don't execute
        **Write to console current value of variable `i`;**

// First loop has ended, i = 3;

Loop from 0 up until it is less than 3 and assign it to variable `j`;
    Call `j`-th function from variable `funcs`:  
        **Write to console current value of variable `i`;**  
        // Ask yourself NOW! What is the value of i?

Now the question is, what is the value of variable i when the function is called? Because the first loop is created with the condition of i < 3, it stops immediately when the condition is false, so it is i = 3.

You need to understand that, in time when your functions are created, none of their code is executed, it is only saved for later. And so when they are called later, the interpreter executes them and asks: "What is the current value of i?"

So, your goal is to first save the value of i to function and only after that save the function to funcs. This could be done for example this way:

var funcs = [];
for (var i = 0; i < 3; i++) {          // let's create 3 functions
    funcs[i] = function(x) {            // and store them in funcs
        console.log("My value: " + x); // each should log its value.
    }.bind(null, i);
}
for (var j = 0; j < 3; j++) {
    funcs[j]();                        // and now let's run each one to see
}

This way, each function will have it's own variable x and we set this x to the value of i in each iteration.

This is only one of the multiple ways to solve this problem.

@Ali Kahoot 2016-11-04 07:46:07

First of all, understand what's wrong with this code:

var funcs = [];
for (var i = 0; i < 3; i++) {          // let's create 3 functions
    funcs[i] = function() {            // and store them in funcs
        console.log("My value: " + i); // each should log its value.
    };
}
for (var j = 0; j < 3; j++) {
    funcs[j]();                        // and now let's run each one to see
}

Here when the funcs[] array is being initialized, i is being incremented, the funcs array is initialized and the size of func array becomes 3, so i = 3,. Now when the funcs[j]() is called, it is again using the variable i, which has already been incremented to 3.

Now to solve this, we have many options. Below are two of them:

  1. We can initialize i with let or initialize a new variable index with let and make it equal to i. So when the call is being made, index will be used and its scope will end after initialization. And for calling, index will be initialized again:

    var funcs = [];
    for (var i = 0; i < 3; i++) {          
        let index = i;
        funcs[i] = function() {            
            console.log("My value: " + index); 
        };
    }
    for (var j = 0; j < 3; j++) {
        funcs[j]();                        
    }
    
  2. Other Option can be to introduce a tempFunc which returns the actual function:

    var funcs = [];
    function tempFunc(i){
        return function(){
            console.log("My value: " + i);
        };
    }
    for (var i = 0; i < 3; i++) {  
        funcs[i] = tempFunc(i);                                     
    }
    for (var j = 0; j < 3; j++) {
        funcs[j]();                        
    }
    

@Alexander Levakov 2016-11-11 09:43:17

Let's take advantage of new Function. Thus i stops to be a variable of a closure and becomes just a part of the text:

var funcs = [];
for (var i = 0; i < 3; i++) {
    var functionBody = 'console.log("My value: ' + i + '");';
    funcs[i] = new Function(functionBody);
}

for (var j = 0; j < 3; j++) {
    funcs[j]();
}

@lovasoa 2016-11-12 10:32:10

This is slow, potentially insecure, and doesn't work everywhere.

@Pawel 2017-02-28 15:09:04

Many solutions seem correct but they don't mention it's called Currying which is a functional programming design pattern for situations like here. 3-10 times faster than bind depending on the browser.

var funcs = [];
for (var i = 0; i < 3; i++) {      // let's create 3 functions
  funcs[i] = curryShowValue(i);
}
for (var j = 0; j < 3; j++) {
  funcs[j]();                      // and now let's run each one to see
}

function curryShowValue(i) {
  return function showValue() {
    console.log("My value: " + i);
  }
}

See the performance gain in different browsers.

@Pawel 2017-12-27 01:52:28

@TinyGiant The example with function being returned is still currying optimised for performance. I wouldn't jump on arrow functions bandwagon like all the JavaScript bloggers. They look cool and clean but promote writing functions inline instead of using predefined functions. This can be a non-obvious trap in hot places. Another problem is that they are not just syntactic sugar because they are executing unnecessary bindings thus creating wrapping closures.

@Tiny Giant 2017-12-27 02:36:38

Warning to future readers: This answer inaccurately applies the term Currying. "Currying is when you break down a function that takes multiple arguments into a series of functions that take part of the arguments.". This code does nothing of the sort. All you've done here is take the code from the accepted answer, move some things around, change the style and naming a bit, then call it currying, which it categorically is not.

@Prithvi Uppalapati 2016-11-07 11:25:54

With new features of ES6 block level scoping is managed:

var funcs = [];
for (let i = 0; i < 3; i++) {          // let's create 3 functions
    funcs[i] = function() {            // and store them in funcs
        console.log("My value: " + i); // each should log its value.
    };
}
for (let j = 0; j < 3; j++) {
    funcs[j]();                        // and now let's run each one to see
}

The code in OP's question is replaced with let instead of var.

@Tiny Giant 2017-12-27 03:05:48

const provides the same result, and should be used when the value of a variable will not change. However, the use of const inside the initializer of the for loop is implemented incorrectly in Firefox and has yet to be fixed. Instead of being declared inside the block, it is declared outside the block, which results in a redeclaration to the variable, which in turn results in an error. The use of let inside the initializer is implemented correctly in Firefox, so no need to worry there.

@jottos 2009-04-15 06:18:29

The reason your original example did not work is that all the closures you created in the loop referenced the same frame. In effect, having 3 methods on one object with only a single i variable. They all printed out the same value.

@Christian Landgren 2014-12-09 22:24:17

I'm surprised no one yet has suggested using the forEach function to better avoid (re)using local variables. In fact, I'm not using for(var i ...) at all anymore for this reason.

[0,2,3].forEach(function(i){ console.log('My value:', i); });
// My value: 0
// My value: 2
// My value: 3

// edited to use forEach instead of map.

@JLRishe 2015-03-31 19:59:43

.forEach() is a much better option if you're not actually mapping anything, and Daryl suggested that 7 months before you posted, so there's nothing to be surprised about.

@jherax 2015-10-27 04:14:23

This question is not about loop over an array

@Christian Landgren 2015-11-11 21:25:03

Well, he wants to create an array of functions, this example shows how to do that without involving a global variable.

@Darren Clark 2009-04-15 06:48:43

Another way of saying it is that the i in your function is bound at the time of executing the function, not the time of creating the function.

When you create the closure, i is a reference to the variable defined in the outside scope, not a copy of it as it was when you created the closure. It will be evaluated at the time of execution.

Most of the other answers provide ways to work around by creating another variable that won't change the value for you.

Just thought I'd add an explanation for clarity. For a solution, personally, I'd go with Harto's since it is the most self-explanatory way of doing it from the answers here. Any of the code posted will work, but I'd opt for a closure factory over having to write a pile of comments to explain why I'm declaring a new variable(Freddy and 1800's) or have weird embedded closure syntax(apphacker).

@neatsu 2017-10-13 08:53:18

Already many valid answers to this question. Not many using a functional approach though. Here is an alternative solution using the forEach method, which works well with callbacks and closures:

let arr = [1,2,3];

let myFunc = (val, index) => { console.log('val: '+val+'\nindex: '+index); };

arr.forEach(myFunc);

@Xufox 2018-01-04 11:04:49

forEach is covered by the accepted answer.

@pixel 67 2016-05-05 11:48:51

And yet another solution: instead of creating another loop, just bind the this to the return function.

var funcs = [];

function createFunc(i) {
  return function() {
    console.log('My value: ' + i); //log value of i.
  }.call(this);
}

for (var i = 1; i <= 5; i++) {  //5 functions
  funcs[i] = createFunc(i);     // call createFunc() i=5 times
}

By binding this, solves the problem as well.

@wpding 2014-07-14 14:42:00

After reading through various solutions, I'd like to add that the reason those solutions work is to rely on the concept of scope chain. It's the way JavaScript resolve a variable during execution.

  • Each function definition forms a scope consisting of all the local variables declared by var and its arguments.
  • If we have inner function defined inside another (outer) function, this forms a chain, and will be used during execution
  • When a function gets executed, the runtime evaluates variables by searching the scope chain. If a variable can be found in a certain point of the chain it will stop searching and use it, otherwise it continues until the global scope reached which belongs to window.

In the initial code:

funcs = {};
for (var i = 0; i < 3; i++) {         
  funcs[i] = function inner() {        // function inner's scope contains nothing
    console.log("My value: " + i);    
  };
}
console.log(window.i)                  // test value 'i', print 3

When funcs gets executed, the scope chain will be function inner -> global. Since the variable i cannot be found in function inner (neither declared using var nor passed as arguments), it continues to search, until the value of i is eventually found in the global scope which is window.i.

By wrapping it in an outer function either explicitly define a helper function like harto did or use an anonymous function like Bjorn did:

funcs = {};
function outer(i) {              // function outer's scope contains 'i'
  return function inner() {      // function inner, closure created
   console.log("My value: " + i);
  };
}
for (var i = 0; i < 3; i++) {
  funcs[i] = outer(i);
}
console.log(window.i)          // print 3 still

When funcs gets executed, now the scope chain will be function inner -> function outer. This time i can be found in the outer function's scope which is executed 3 times in the for loop, each time has value i bound correctly. It won't use the value of window.i when inner executed.

More detail can be found here
It includes the common mistake in creating closure in the loop as what we have here, as well as why we need closure and the performance consideration.

@wpding 2017-04-26 14:19:07

We rarely write this code sample in real, but I think it serves a good example to understand the fundamental. Once we have the scope in mind and how they chained together, it's more clear to see why other 'modern' ways like Array.prototype.forEach(function callback(el) {}) naturally works: The callback that's passed in naturally forms the wrapping scope with el correctly bound in each iteration of forEach. So every inner function defined in callback will be able to use the right el value

@Costa 2016-12-07 17:33:27

JavaScript functions "close over" the scope they have access to upon declaration, and retain access to that scope even as variables in that scope change.

var funcs = []

for (var i = 0; i < 3; i += 1) {
  funcs[i] = function () {
    console.log(i)
  }
}

for (var k = 0; k < 3; k += 1) {
  funcs[k]()
}

Each function in the array above closes over the global scope (global, simply because that happens to be the scope they're declared in).

Later those functions are invoked logging the most current value of i in the global scope. That's the magic, and frustration, of closure.

"JavaScript Functions close over the scope they are declared in, and retain access to that scope even as variable values inside of that scope change."

Using let instead of var solves this by creating a new scope each time the for loop runs, creating a separated scope for each function to close over. Various other techniques do the same thing with extra functions.

var funcs = []

for (let i = 0; i < 3; i += 1) {
  funcs[i] = function () {
    console.log(i)
  }
}

for (var k = 0; k < 3; k += 1) {
  funcs[k]()
}

(let makes variables that are block scoped instead of function scoped. Blocks are denoted by curly braces, but in the case of the for loop the initialization variable, i in our case, is considered to be declared in the braces.)

@Modermo 2017-04-05 02:50:29

I struggled to understand this concept until I read this answer. It touches on a really important point – the value of i is being set to the global scope. When the for loop finishes running, the global value of i is now 3. Therefore, whenever that function is invoked in the array (using, say funcs[j]), the i in that function is referencing the global i variable (which is 3).

@Rax Wunter 2015-12-17 14:14:56

I prefer to use forEach function, which has its own closure with creating a pseudo range:

var funcs = [];

new Array(3).fill(0).forEach(function (_, i) { // creating a range
    funcs[i] = function() {            
        // now i is safely incapsulated 
        console.log("My value: " + i);
    };
});

for (var j = 0; j < 3; j++) {
    funcs[j](); // 0, 1, 2
}

That looks uglier than ranges in other languages, but IMHO less monstrous than other solutions.

@Quentin 2015-12-17 14:24:19

Prefer it to what? This seems to be a comment in reply to some other answer. It doesn't address the actual question at all (since you aren't assigning a function, to be called later, anywhere).

@Rax Wunter 2015-12-17 14:28:45

Now it's clear?

@Rax Wunter 2015-12-17 14:31:00

It's related exactly to the mentioned issue: how to iterate safely without closure problems

@Quentin 2015-12-17 14:31:10

Now it doesn't seem significantly different from the accepted answer.

@Rax Wunter 2015-12-17 14:34:33

No. In the accepted answer it is suggested to use "some array", but we deal with a range in the answer, it's absolutely different things, which unfortunately don't have a good solution in js, so my answer is trying to solve the issue in a good and practice way

@Rax Wunter 2015-12-17 14:45:00

@Quentin I would recommend to investigate solution before minusing

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