How does this JavaScript/jQuery syntax work: (function( window, undefined ) { })(window)?

Question

Have you ever taken a look under the hood at the jQuery 1.4 source code and noticed how it's encapsulated in the following way:

(function( window, undefined ) {

  //All the JQuery code here 
  ...

})(window);

I've read an article on JavaScript Namespacing and another one called "An Important Pair of Parens," so I know some about what's going on here.

But I've never seen this particular syntax before. What is that undefined doing there? And why does window need to be passed and then appear at the end again?

Leta

I wanted to add that Paul Irish talks about this in this video: paulirish.com/2010/10-things-i-learned-from-the-jquery-source – Haemophilia

@Bergi you marked my question as a duplicate of another but I asked the question almost a year before the duplicate. The cast should be the other way around. – Leta

@dkinzer: It's not about being asked earlier, it's about highest-quality answer. Admittedly, in this case it's neck and neck, but I found CMS' answer to be the best one. Come to chat.stackoverflow.com/rooms/17 to discuss this, though – Dowling

See also What advantages does using (function(window, document, undefined) { … })(window, document) confer? – Dowling

Answer 1

The undefined is a normal variable and can be changed simply with undefined = "new value";. So jQuery creates a local "undefined" variable that is REALLY undefined.

The window variable is made local for performance reasons. Because when JavaScript looks up a variable, it first goes through the local variables until it finds the variable name. When it's not found, JavaScript goes through the next scope etc. until it filters through the global variables. So if the window variable is made local, JavaScript can look it up quicker. Further information: Speed Up Your JavaScript - Nicholas C. Zakas

Tern

Thanks! that was a very informative video. I think you need to edit your response to say "the window variable is made local". I do think this is the best answer. I counted and found that the window object is called at least 17 times in the JQuery source code. So there's got to be a significant effect in moving the window into the Local Scope. – Leta

@DKinzer Oh yes, you're rght, of course it's made local. Glad I could help you =) – Tern

According to this updated test jsperf.com/short-scope/5 passing 'window' is actually slower, when it come to get a window constant through jQuery, and particularly bad for Safari. So while 'undefined' has a use case, I am not quite convinced that 'window' is particularly useful in all cases. – Expectancy

It has also a positive effect for minifying the code. So every usage of "window" and "undefined" can be replaced with a shorter name by the minifyer. – Rintoul

@Leta do you have any statistics that backs your significance? – Counterpoison

@lukas.pukenis When you're making a library that's used in millions of websites, it's wise not to make assumptions about what madmen will do. – Er

As per ECMAScript 5, now undefined is not writable,configurable so its not an issue anymore in modern browsers even if its not strict. Can you please mention this in your answer.. – Thagard

Answer 2

Undefined

By declaring undefined as an argument but never passing a value to it ensures that it is always undefined, as it is simply a variable in the global scope that can be overwritten. This makes a === undefined a safe alternative to typeof a == 'undefined', which saves a few characters. It also makes the code more minifier-friendly, as undefined can be shortened to u for example, saving a few more characters.

Window

Passing window as an argument keeps a copy in the local scope, which affects performance: http://jsperf.com/short-scope. All accesses to window will now have to travel one level less up the scope chain. As with undefined, a local copy again allows for more aggressive minification.

---

Sidenote:

Though this may not have been the intention of the jQuery developers, passing in window allows the library to be more easily integrated in server-side Javascript environments, for example node.js - where there is no global window object. In such a situation, only one line needs to be changed to replace the window object with another one. In the case of jQuery, a mock window object can be created and passed in for the purpose of HTML scraping (a library such as jsdom can do this).

Crenel

+1 for mentioning Minification , wish I could +2 for the jsperf -- The minified version is (function(a,b){})(window); -- a and b are much shorter than window and undefined :) – Spay

+1 I had heard of scope chain before, but forgot the term. Thanks! – Vaginate

It's also an alternative to using void(0) which was meant for the same reason: void(0) is a safe way to get the undefined value. – Loaiasis

"What you say" is in reference to this other question: #4945765 – Spay

@gnarf, thanks for the notification that the questions were merged. I'll edit my answer to make a little more sense ;) – Crenel

btw, the global window.undefined can be overwritten in ECMAScript3 only, the ESCMASCript5 doesn't allow you this and the most of browsers run under ECMAScript5 standard nowadays. I guess IE7 allows to override the undefined global. IE8+ is OK. – Venita

So wouldn't var window ={}; will not do same trick in window less script? – Counterpoison

You can pass this to the IIFE instead of window to get the global scope in both browser and node.js environments. Example: (function (root, undefined) { 'use strict'; var $ = root.jQuery; }(this)); – Heartbreaking

Answer 3

Others have explained undefined. undefined is like a global variable that can be redefined to any value. This technique is to prevent all undefined checks from breaking if someone wrote say, undefined = 10 somewhere. An argument that is never passed is guaranteed to be real undefined irrespective of the value of the variable undefined.

The reason to pass window can be illustrated with the following example.

(function() {
   console.log(window);
   ...
   ...
   ...
   var window = 10;
})();

What does the console log? The value of window object right? Wrong! 10? Wrong! It logs undefined. Javascript interpreter (or JIT compiler) rewrites it this way -

(function() {
   var window; //and every other var in this function

   console.log(window);
   ...
   ...
   ...
   window = 10;

})();

However, if you get the window variable as an argument, there is no var and hence no surprises.

I don't know if jQuery is doing it, but if you are redefining window local variable anywhere in your function for whatever reason, it is a good idea to borrow it from global scope.

Rasbora

Answer 4

window is passed in like that just in case someone decides to redefine the window object in IE, I assume the same for undefined, in case it's re-assigned in some way later.

The top window in that script is just naming the argument "window", an argument that's more local that the global window reference and it what the code inside this closure will use. The window at the end is actually specifying what to pass for the first argument, in this case the current meaning of window...the hope is you haven't screwed up window before that happens.

This may be easier to think of by showing the most typical case used in jQuery, plugin .noConflict() handling, so for the majority of code you can still use $, even if it means something other than jQuery outside this scope:

(function($) {
  //inside here, $ == jQuery, it was passed as the first argument
})(jQuery);

Yeasty

Thanks. This makes a lot of sense. Though, I think the answer is a combination of this and what @C.Zakas says. – Leta

@DKinzer I'm afraid I'm not C. Zakas ;) – Tern

@Vincent, sorry about that :-) – Leta

Answer 5

Tested with 1000000 iterations. This kind of localization had no effect in performance. Not even a single millisecond in 1000000 iterations. This is simply useless.

Hodometer

Not to mention, closure carries all variable along with instance of function, so if you have 100 bytes in closure, and 1000 instances that is 100kb of more memory. – Counterpoison

@AkashKava and @Semra, i don't think this test really accounts for why you would pass in window in a real world situation. The performance gain is only going to be seen when you have deeply nested closures that access that variable further along the scope chain. obviously if your function is only one step away on the scope chain from the variable, you won't see much gain. but if it was waaaay down there and needed to get window you might see some gain by a local copy. – Tombstone

@Tombstone new JavaScript engines resolves closures at time of compilation itself, there is no performance gain in closure at runtime. I doubt there is any measurable performance gain, if you are so confident, you can create jsperf example to demonstrate performance. The only performance we get is by isolating closures which results in better minification which reduces size and performance is achieved by faster download and parsing of script. – Counterpoison

@AkashKava what makes you think i'm so confident? I just said "might see some gain" and "i don't think". I could create a test, but i'd rather not since i never use this pattern anyway. can you explain what you mean by "resolves closures at time of compilation itself"? as opposed to what alternative? how did Javascript engines do things differently before? – Tombstone

Have a find a code where window is curried at the end but not entered in the function's parameters, what does mean? it ensure that the parameters follow the original global's scope object even if there isn't any window param I assume? – Audacious