I was wondering, how does the $. in $.ajax({...}); work? it doesnt make sense to me. Sure .ajax as a member make sense but $ isnt a variable name? or is it? How is it defined?

I was wondering, how does the $. in $.ajax({...}); work? it doesnt make sense to me. Sure .ajax as a member make sense but $ isnt a variable name? or is it? How is it defined?

• javascript
• jquery
• syntax

• 6 It's a function name, have a read of the jquery source. The function effectively returns a JQuery object which has the member 'ajax'. – Lazarus Commented Sep 22, 2009 at 12:16
• 1 @Lazarus: actually, though the $ function does return a jQuery object when called, that's not what's going on here. $, like all functions in JS, is an object, and can have its own properties (i.e. ajax). the jQuery object returned when calling $("") does not have an ajax property (try typeof $("").ajax as opposed to typeof $.ajax) – maltalef Commented Sep 23, 2009 at 14:44

6 Answers 6

$ is the same as jQuery. That is, you can write jQuery.ajax(...) etc.

The confusing part is $ is a legal character in Javascript variable names. It doesn't have any special meaning, as it does in PHP or Perl, for instance.

From the source:

// Map over jQuery in case of overwrite
_jQuery = window.jQuery,
// Map over the $ in case of overwrite
_$ = window.$,

jQuery = window.jQuery = window.$ = function( selector, context ) {
// The jQuery object is actually just the init constructor 'enhanced'
    return new jQuery.fn.init( selector, context );
},

It's a function (first-class object) with properties, such as the ajax function you mention.

"$" is a valid character for variable names, and as you can see from the code snippet, $ is the same as jQuery.

$ This is defined within the jQuery library to be a reference to jQuery shorter. You can download the library and see the first lines:

var
    // Will speed up references to window, and allows munging its name.
    window = this,
    // Will speed up references to undefined, and allows munging its name.
    undefined,
    // Map over jQuery in case of overwrite
    _jQuery = window.jQuery,
    // Map over the $ in case of overwrite
    _$ = window.$,

This "window.$", "$" Belongs to the window object environment.

As explained in a number of JavaScript resources, including Mozilla's JavaScript Guide:

A JavaScript identifier must start with a letter, underscore (_), or dollar sign ($); subsequent characters can also be digits (0-9). Because JavaScript is case sensitive, letters include the characters "A" through "Z" (uppercase) and the characters "a" through "z" (lowercase).

So the following are all legal (though ill-advised) in JavaScript:

var $ = function() {};

var ____ = 0;

var __$__$ = 1;

function _$_$_$_(_, __, $_$) {
    return (_ * __) + $_$;
}

alert(_$_$_$_(3,2,1)); // shows 7 

$ is one of the only legal characters that can be used in Javascript variable names. JQuery and other libraries take advantage of that initializing $ to be a function that initializes the jQuery object.

If I remember correctly the code looks somewhat like the following:

$ = window.jQuery = function(){
    return new jQuery(args);
}

In javascript, functions are objects (that can be contained by variables). As such, they can have properties (and methods which are just properties with functions as values). Try this:

function test () {
    alert("hey!");
}
test.foo = function (msg) {
    alert("you said: "+msg);
};

test(); //alerts "hey!"
test.foo("123") //alerts ""you said: 123".

//note that I'm not calling test().foo(),
//as test() returns nothing, though it could
//return an object (with a foo() or any other method itself!)

this is sort of what happens with jQuery.