What is this JavaScript

Question

What would the following code do?

I have used JS for years but haveno idea how this construct works?

(function() { /* No implementation yet */ })();

Knowing that there is no implementation - if there were - how would I invoke it? Would the following make an anonymous object?

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

Which I could use as:

temp.doWhateverDefined();

You cannot unless you returns some function (doWhateverDefined) from that function. — Selvakumar Arumugam, May 10 '13 at 17:21
It's already being invoked. It's meant as a one-time-use function to create a local variable scope. — , May 10 '13 at 17:22
it is an anonymous function, which is called just after it's defined.... — Piyuesh, May 10 '13 at 17:22

score 4 · Answer 1 · answered May 10 '13 at 17:21

4

You have an immediately invoked function expression (IIFE). It is very common in JavaScript.

answered May 10 '13 at 17:21

Evan Davis

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Thats kinda cool it's like inline code that is executed but in a function scope - but without explicitly calling any function??? – Alex.Barylski May 10 '13 at 17:31
@Alex.Barylski it explicitly calls the anonymous function wrapping the code. In JavaScript, you invoke a function by adding parens, `()`. This defines a function, and then immediately calls it by appending `()` to the end. Worth noting, the final `()` can be inside OR outside the wrapping parens. `(function() {}())` is just as good as `(function() {})()` – Evan Davis May 10 '13 at 17:34

score 2 · Answer 2 · answered May 10 '13 at 17:23

2

(function() { /* No implementation yet */ })();

This is called an Immediately Invoked Function Expression or shortly IIFE. It is declared, evaluated and called immediately.

The basic idea is:

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

answered May 10 '13 at 17:23

flavian

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2

Or *IEFE*? I guess that works too. ;-) – May 10 '13 at 17:26

score 1 · Answer 3 · answered May 10 '13 at 17:23

It's being run immediately. It's as if you said:

var f = function() { /* No implementation yet */ }
(f)();

which is the same as

var f = function() { /* No implementation yet */ }
f();

The point of it is to allow a block of code (page initialization, etc.) to use whatever variable/function names it likes, without conflicting with other Javascript code that may use the same names. All the functions/etc. declared in that block are local, and don't harm the outside world.

score 0 · Answer 4 · answered May 10 '13 at 17:21

That is an anonymous function so you cannot invoke other than in the line where you create it unless you assign it to a name

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

This piece of code should be changed to

var temp = (function() {  });

That way you actually assign the function to the name. To invoke it just use a parentheses

temp()

score 0 · Answer 5 · answered May 10 '13 at 17:22

0

It is a self-executing function. You do not invoke it, it does it automatically.

var temp = (function() { })(); would work if the function returns something.

answered May 10 '13 at 17:22

gpojd

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score 0 · Answer 6 · answered May 10 '13 at 17:26

0

What you have is a self-invoking function which is already called.

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

So the function has to returns a function so that temp() or temp.somethin(); could be triggered. See an example below for more information,

var temp = (function() { return {
    doWhateverDefined: function () {
        return 'invoked';}
      };  
    }
)();

alert(temp.doWhateverDefined()); //should alert invoked

DEMO: http://jsfiddle.net/5ch8F/1/

answered May 10 '13 at 17:26

Selvakumar Arumugam

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Thanks all for the replies...I kinda figured it was called immediately after being defined but I appreciate the IIFE blog thats awesome :) – Alex.Barylski May 10 '13 at 17:29

What is this JavaScript

6 Answers6