JavaScript Class Definition Starts with (function()

Question

Check source code of ApiValidator.js at github. I was taught to define a JS class using var apple = new function() {... But this one starts with (function(){....? Why is this?

Neither `new function() { ...` nor `(function() { ...` are ways to define a class in JavaScript. — Pointy, Sep 09 '15 at 20:43
Unless you are using ecmascript 6 there is no such thing as a class in javascript. What you are defining is functions. — bhspencer, Sep 09 '15 at 20:46

thecodeparadox · Answer 1 · 2015-09-09T20:50:40.910

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First of all, var apple = new function() {} is not a way of define a class.

and (function () {...})() is a self-executing function that calling itself immediately.

For example: var fn = function () { // code };

so fn() and (function() { // code })() is similar.

edited Sep 09 '15 at 20:50

answered Sep 09 '15 at 20:45

thecodeparadox

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score 0 · Accepted Answer · edited May 23 '17 at 11:58

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The code in the GitHub starts with and ends with:

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

This is called as a self-executing function or "Immediately-Invoked Function Expression" (IIFE). This encloses an anonymous function inside it and immediately executes it. And one more thing:

var apple = new function() {
}

The above is not a valid way of defining an object or a class.

edited May 23 '17 at 11:58

Community

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answered Sep 09 '15 at 20:45

Praveen Kumar Purushothaman

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well `new function() { ... }` definitely will create an *object*, but I don't think too many people would say it creates a "class". – Pointy Sep 09 '15 at 20:46
That is NOT a closure. It's a self-executing function. A closure is only defined when something is being held on to that would have otherwise been garbage collected but can't because of a reference inside of some other function (a closure). – dmeglio Sep 09 '15 at 20:47
@Pointy Oops... Sorry about my mistake. How do I correct it? Should I remove it? – Praveen Kumar Purushothaman Sep 09 '15 at 20:47
@PraveenKumar maybe just edit the answer? – Pointy Sep 09 '15 at 20:47
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Technically it is called an "Immediately-Invoked Function Expression" (IIFE) http://stackoverflow.com/questions/8228281/what-is-the-function-construct-in-javascript – bhspencer Sep 09 '15 at 20:48
@Pointy Grr... What should the [edit] contain! `:P` – Praveen Kumar Purushothaman Sep 09 '15 at 20:48
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I wouldn't bother editing it. This question has been asked many times before and will almost certainly be closed. – bhspencer Sep 09 '15 at 20:49
Done it. Have a look. – Praveen Kumar Purushothaman Sep 09 '15 at 20:50
@PraveenKumar well I guess you could say that it's a (somewhat unusual) way of instantiating an object. I guess it might be a useful way of building an object with a single expression when you can't use an object initializer for whatever reason. – Pointy Sep 09 '15 at 21:00

JavaScript Class Definition Starts with (function()

2 Answers2