Javascript: closure of loop?

Question

I would like to do the something along the following:

for (var i = 0; i < 10; ++i) {
 createButton(x, y, function() { alert("button " + i + " pressed"); }
}

The problem with this is that I always get the final value of i because Javascript's closure is not by-value.
So how can I do this with javascript?

• user657496: You could edit createButton, allowing it to have another argument passed, that is, i. This way you can store i in your createButton function and use it.
• rds: possible duplicate of Javascript closure inside loops - simple practical example

Answer 1

One solution, if you're coding for a browser that uses JavaScript 1.7 or higher, is to use the let keyword:

for(var i = 0; i < 10; ++i) {
 let index = i;
 createButton(x, y, function() { alert("button " + index + " pressed"); }
}

From the MDC Doc Center:

The let keyword causes the item variable to be created with block level scope, causing a new reference to be created for each iteration of the for loop. This means that a separate variable is captured for each closure, solving the problem caused by the shared environment.

Check out the MDC Doc Center for the traditional approach (creating another closure).

Answer 2

for(var i = 0; i < 10; i++) {
 (function(i) {
 createButton(function() { alert("button " + i + " pressed"); });
 })(i);
}

Note that JSLint doesn't like this pattern. It throws "Don't make functions within a loop.".

Live demo: http://jsfiddle.net/simevidas/ZKeXX/

• McStretch: I like this answer more than Peter's due to its cleanliness. It would be nice if more browsers supported the let keyword though.
• Šime Vidas: @McStretch I tried to make a let demo in jsFiddle, but I couldn't make it work. See here: jsfiddle.net/simevidas/ZKeXX/1 Firefox 4 throws an error.
• McStretch: @Šime - I just saw this question: stackoverflow.com/questions/2356830/… , which says you have to explicitly tell the browser (Firefox only right now) that you're using 1.7. Here's an updated fiddle: jsfiddle.net/simevidas/ZKeXX/1. Pretty lame huh? Obviously not really a good solution until everyone supports 1.7 or higher, and who know when that will happen.
• Šime Vidas: @McStretch I see. The updated demo (which works in Firefox) is here: jsfiddle.net/simevidas/ZKeXX/2 It seems that we won't be able to use let for a long time. Even if IE10 implements it, we'd have to wait until IE9 exits the market (and that may not happen before 2020).

Answer 3

Create a new scope for the closure by executing another function:

for(var i = 0; i < 10; ++i) {
 createButton(x,y, function(value) { return function() { alert(...); }; }(i));
}

http://www.mennovanslooten.nl/blog/post/62

• Šime Vidas: 1. You might want to place a semi-colon at the end of the return statement to make the code a bit more readable. 2. IIFE's are usually wrapped in parens.

Answer 4

You need to put the closure into a separate function.

for(var dontUse = 0; dontUse < 10; ++dontUse) {
 (function(i) {
 createButton(x, y, function() { alert("button " + i + " pressed"); }
 })(dontUse);
}

Thise code creates an anonymous function that takes i as a parameter for each iteration of the loop.
Since this anonymous function has a separate i parameter for each iteration, it fixes the problem.

This is equivalent to

function createIndexedButton(i) {
 createButton(x, y, function() { alert("button " + i + " pressed"); }
}
for(var i = 0; i < 10; ++i) {
 createIndexedButton(i);
}

Answer 5

for(var i = 0; i < 10; ++i) {
 createButton(x, y, (function(n) {
 return function() {
 alert("button " + n + " pressed");
 }
 }(i));
}

The anonymous function on the outside is automatically invoked and creates a new closure with n in its scope, where that takes the then current value of i each time it's invoked.