JavaScript () following }

Question

I am studying JavaScript and I sometimes see something like this

function functionname()
{
   // Some statements
} () ;

What does () following } mean?

Thanks a lot, experts on Stack Overflow

Well, the intention is probably to call the function immediately as the answers discuss. But, note that it's a SyntaxError in this snippet. A [function declaration](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function) as this appears to be cannot be followed immediately by `()`. However, a [function expression](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/function) can (thus the `FE` in IIFE). This distinction is why there's often another set of parenthesis around the `function`, which force it to be an expression. — Jonathan Lonowski, Sep 16 '13 at 21:27

tckmn · Answer 1 · 2013-09-16T21:26:47.917

8

That is an IIFE (immediately invoked function expression). It is used to create a new scope. For example:

var x = 10;
(function() {
   var x = 5;
}());
alert(x); // still 10

All it does is define and call the function at the same time.

IIFEs are also used to "save" the value of a variable. For example, this code won't work:

for (var i = 0; i < buttons.length; i ++) {
    buttons[i].onclick = function() { alert(i) }
}

Every button will alert the last index when you click it, because after the loop is done i is buttons.length. To fix it, you would use an IIFE:

for (var i = 0; i < buttons.length; i ++) {
    buttons[i].onclick = (function(i) {
        return function() {
            alert(i)
        }
    })(i)
}

More info on IIFEs

edited Sep 16 '13 at 21:26

answered Sep 16 '13 at 21:19

tckmn

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More info on this. http://benalman.com/news/2010/11/immediately-invoked-function-expression/ – Tomislav Markovski Sep 16 '13 at 21:21
@Doorknob I have another question. What would be the benefit of using this kind of function since it won't be reusable? (not callable from somewhere else) – Caner Akdeniz Sep 16 '13 at 21:51
@CanerAkdeniz If you only need to use it once. – tckmn Sep 16 '13 at 21:54
@Doorknob then why would i need to create a function? I could do it without creating a function. Is there any performance benefit? – Caner Akdeniz Sep 17 '13 at 14:21
@Caner To create a new scope, as I said in my answer. – tckmn Sep 17 '13 at 17:22

score 7 · Answer 2 · answered Sep 16 '13 at 21:19

7

It calls the function.

var foo = function () { return 1; }();
alert(foo); // 1

answered Sep 16 '13 at 21:19

Quentin

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rafaelcastrocouto · Answer 3 · 2013-09-17T12:20:46.527

0

There are lots of ways to define a function, when you do it like this ...

(function functionname(innerArguments){
   // Some statements;
})(outerArguments);

... the function is executed as soon as this piece of code is interpreted; its the same as:

function functionname(innerArguments){
   // Some statements;
};
functionname(outerArguments);

edited Sep 17 '13 at 12:20

answered Sep 16 '13 at 21:25

rafaelcastrocouto

11,781
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Note that these snippets aren't actually identical; the first one is interpreted as a function declaration and then a useless expression. Your first snippet defines the function but does not execute it. – tckmn Sep 16 '13 at 21:29

JavaScript () following }

3 Answers3