Stop passing by reference in closures

Question

I have code that looks like this:

var a = [];
for(var i = 0; i < 10; i++) {
    a[i] = function() {
        console.log(i);
    }
}

Unfortunately, it seems that i is being passed by reference, so all the functions in a output 10. How do I make it so that each function outputs the value that i had when it was created? I.e. a[0]() gives 0, a[1]() gives 1, etc.

EDIT: to clarify, I do not want a to store the values 0-9. I want a to store functions that return the values 0-9.

Classic "closure in a loop" error. – Amadan Aug 25 '15 at 00:37 — Amadan, Aug 25 '15 at 00:37

score 5 · Accepted Answer · answered Aug 25 '15 at 00:35

5

You need to invoke a function (to create a closure that captures your value) which returns a function (the one you want to end up with). Something like this:

var a = [];
for (var i = 0; i < 10; i++) {
    a[i] = (function(value) {
        return function() {
            console.log(value);
        }
    })(i);
}

answered Aug 25 '15 at 00:35

Ghazgkull

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Functionception. I like it! – Bluefire Aug 25 '15 at 00:37
Yeah, pretty much. Once you wrap your head around it, the whole closure thing is pretty damn cool. – Ghazgkull Aug 25 '15 at 00:38

MD Sayem Ahmed · Answer 2 · 2015-08-25T00:35:56.897

0

var a = [];
for(var i = 0; i < 10; i++) {
    a[i] = (function(j) {
        return function () {
            console.log(j);
        }
    })(i);
}

A better performing version -

function makeFunction(i) {
    return function () {
        console.log(i);
    }
}

var a = [];
for(var i = 0; i < 10; i++) {
    a[i] = makeFunction(i);
}

JSFiddle Demo.

edited Aug 25 '15 at 00:35

answered Aug 25 '15 at 00:28

MD Sayem Ahmed

28,628
27
111
178

The first one will not work. It will execute the function immediately, rather than store it in `a` for later execution. – Bluefire Aug 25 '15 at 00:32
@Bluefire: I believe now it does what you need. Sorry I did not notice the function storing part. – MD Sayem Ahmed Aug 25 '15 at 00:36

Stop passing by reference in closures

2 Answers2