Wrapping a file with (function() { … }).call(this) versus a call with simply ()

Question

The backbone.js source code uses a function wrapper like this:

(function(){
    ...
}).call(this);

as seen at http://backbonejs.org/docs/backbone.html#section-185.

Much more often, I've seen the following used instead:

(function(){
    ...
})();

When does the behavior of these two differ? I was under the impression that they were equivalent, but I assume that there must be a difference given that Backbone uses .call(this) instead of the shorter alternative.

Corbin · Accepted Answer · 2012-07-20T23:21:02.903

8

In the first example, this inside of the function will be the this from the calling scope.

In the second example, this will be window.

(As noted by Šime Vidas, it's undefined in strict mode, rather than window.)

edited Jul 20 '12 at 23:21

answered Jul 20 '12 at 22:59

Corbin

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Oops, you're absolutely right. Thanks! – Sophie Alpert Jul 20 '12 at 23:04
1

One can easily verify this by evaluating `(function() { return (function() { return this; })(); }).call(2)` and noting that it evaluates to `window`, not `2`. – Sophie Alpert Jul 20 '12 at 23:05
It's worth mentioning that in strict mode it's `undefined` rather than the global object. – Šime Vidas Jul 20 '12 at 23:17
@ŠimeVidas To be honest, I actually did not know that (makes sense though). Will edit the answer to note that. Thanks. – Corbin Jul 20 '12 at 23:20

score 4 · Answer 2 · answered Jul 20 '12 at 22:59

4

With the first this is the same in the function as it is in the scope when it is called, for the second this would refer to the window object.

answered Jul 20 '12 at 22:59

Musa

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Wrapping a file with (function() { … }).call(this) versus a call with simply ()

2 Answers2