Declaration function inside ( )

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What advantages does using (function(window, document, undefined) { … })(window, document) confer?
Advanced Javascript: Why is this function wrapped in parentheses?

I just checked how jquery is written, then in the first line of it i see this:

(function( window, undefined ) {

});

My question is what is the meaning or reason the declaration of function is inside the ( and )?

http://benalman.com/news/2010/11/immediately-invoked-function-expression/ — SLaks, Jan 22 '13 at 15:46
http://stackoverflow.com/questions/8228281/what-is-this-construct-in-javascript — zzzzBov, Jan 22 '13 at 15:48
See: http://stackoverflow.com/a/14318019/613004 for a decent explanation of why it's used. — Faust, Jan 22 '13 at 15:49
also: http://stackoverflow.com/questions/9053842/advanced-javascript-why-is-this-function-wrapped-in-parentheses — zzzzBov, Jan 22 '13 at 15:50
The grouping operator (`(...)`) forces whatever is inside to be evaluated as an expression. — Felix Kling, Jan 22 '13 at 15:52

score 4 · Accepted Answer · answered Jan 22 '13 at 15:49

In your example, I see no reason for the parentheses.

For immediately invoked functions, Douglas Crockford recommends and provides a code sample as below. Source is http://javascript.crockford.com/code.html

When a function is to be invoked immediately, the entire invocation expression should be wrapped in parens so that it is clear that the value being produced is the result of the function and not the function itself.

var collection = (function () {
    var keys = [], values = [];

    return {
        get: function (key) {
            var at = keys.indexOf(key);
            if (at >= 0) {
                return values[at];
            }
        },
        set: function (key, value) {
            var at = keys.indexOf(key);
            if (at < 0) {
                at = keys.length;
            }
            keys[at] = key;
            values[at] = value;
        },
        remove: function (key) {
            var at = keys.indexOf(key);
            if (at >= 0) {
                keys.splice(at, 1);
                values.splice(at, 1);
            }
        }
    };
}());

score 2 · Answer 2 · answered Jan 22 '13 at 15:49

2

actually, the first line is like this:

(function( window, undefined ) {

})( window );

which is an immediately-invoked function expression.

answered Jan 22 '13 at 15:49

ic3b3rg

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score 1 · Answer 3 · answered Jan 22 '13 at 15:49

You typed it wrong, it was probably written:

(function(...) {

    // script

})(...);

It is used to avoid conflicts. This is a self-invoking function. Because of the parentheses at the end, it invokes itself.

All variables, objects and functions defined within this function, stays in this function.

score 1 · Answer 4 · answered Jan 22 '13 at 15:52

1

See: http://www.ecma-international.org/ecma-262/5.1/#sec-12.4

This is just part of the ECMAScript (and thus JavaScript) syntax.

You need the parens to call the anonymous function immediately.

function () {}() causes a syntax error. Either of these work, though:

(function () {}())
(function () {})()

The main point is to wrap all of the function's contents so that external variables do not leak into the function (and vice versa). It is anonymous so that there is no global function declaration added either.

answered Jan 22 '13 at 15:52

Explosion Pills

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it isn't even a declaration, but an expression – Moritz Roessler Jan 22 '13 at 15:54
@Glutamat what I meant was that there is no declaration, so no global function is added – Explosion Pills Jan 22 '13 at 15:56

score 0 · Answer 5 · answered Jan 22 '13 at 15:55

As SLaks already linked, it's a self-executing-function. For the 2 parameters inside is another reason, performance and security.

performance wise, javascript will look into itself for the variable, else he will check his parents and so on. As u can imagine, local variables will be always the quickest.

For the undefined section, here is an example: A hacker can just say;

undefined = null;

and now your code is failing and will have bugs and stuff.

I hope this was helpfull.

Declaration function inside ( )

5 Answers5