Purpose of (fn...)() brackets in Self-Executing Anonymous Functions? [JavaScript]

Question

Possible Duplicate:
Location of parenthesis for auto-executing anonymous JavaScript functions?

Question is a duplicate of Location of parenthesis for auto-executing anonymous JavaScript functions? and What do parentheses surrounding a object/function/class declaration mean?

Just curious really, what are the purposes of the brackets in this code:

(function() {})();

This looks like I could just as easily write:

var x=function(){};
(x)();

With jQuery plugins we would do something like...

(function($) {})(jQuery);

What's the deal with the brackets?

possible duplicate of [Location of parenthesis for auto-executing anonymous JavaScript functions?](http://stackoverflow.com/questions/3384504/location-of-parenthesis-for-auto-executing-anonymous-javascript-functions) and of [Is there a difference between (function() {…}()); and (function() {…})(); ?](http://stackoverflow.com/questions/3783007/is-there-a-difference-between-function-and-function) — Peter Ajtai, Oct 06 '10 at 18:56
@Gumbo: yes, found [a duplicate](http://stackoverflow.com/questions/440739/what-do-parentheses-surrounding-a-javascript-object-function-class-declaration-me) (already a duplicate as well). — palswim, Oct 06 '10 at 18:56
Thanks for pointing it out. Please close this. In Canada we don't commonly use the term parenthesis to refer to any of these charecters: () {} [], which get called brackets (eg, we call it BEDMAS instead of PEDMAS) — Incognito, Oct 06 '10 at 18:59
@user257493 - [ **Wikipedia expounds on brackets.** ](http://en.wikipedia.org/wiki/Bracket) — Peter Ajtai, Oct 06 '10 at 19:02

palswim · Accepted Answer · 2010-10-06T19:05:50.787

2

With the parentheses (surrounding the function), you don't have to declare a name, littering the namespace. And, they serve to alert readers of your code to the fact that you're using a self-invoking function.

The second set of parentheses actually invoke/call the (anonymous) function you just created. Since Javascript functions are actually just variables (or "first-class objects" in CS-speak), you've just created a variable (in the first set of parentheses), which you call using the second set.

Here's an example:

function callFunc(f) {
    return f("test");
}

callFunc(alert);

In the example, f actually references the function alert, which you call in the function code with the parentheses.

edited Oct 06 '10 at 19:05

answered Oct 06 '10 at 18:54

palswim

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Why do I almost always see a set of brackets right after the part that encapsulates the function? Did you happen to know the ecma section that describes this behavior? – Incognito Oct 06 '10 at 18:56
@user257493: those parentheses actually invoke the function. – Daniel Sloof Oct 06 '10 at 18:58
as a note: one parenthesis, two parentheses. – Peter Ajtai Oct 06 '10 at 19:04

score 1 · Answer 2 · answered Oct 06 '10 at 18:55

1

In case of the jQuery example you define an anonymous function that takes a parameter called $ and then pass the jQuery object to it. It will stay inside that scope and not conflict with other frameworks that have $ globally defined.

Other than that, personally it looks cleaner to me.

answered Oct 06 '10 at 18:55

Daniel Sloof

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I'm not really concerned with what the jQuery plugin code does as much as I am the purposes of the brackets. – Incognito Oct 06 '10 at 18:57
That's more the reason for enclosing the code in a function, and not so much a reason for the parentheses themselves. – palswim Oct 06 '10 at 19:07

Purpose of (fn...)() brackets in Self-Executing Anonymous Functions? [JavaScript]

2 Answers2