The Structure of jQuery

• jQuery Introduction
• Sizzle
• init and jQuery prototype
• Method Chaining
• Expose Globally

We examine famous frameworks and libraries,such as jQuery,to experience the open source education.This article mainly analyses the structure of jQuery.

Course link: JavaScript: Understanding the Weird Parts

(Section 8: Examining Famous Frameworks and Libraries)

jQuery Introduction

jQuery is just a JavaScript library.What jQuery does essentially is let you manipulate the DOM.It handles the browser quirks but doesn’t add any features to the browser or to JavaScript itself.

the jQuery function isn’t a function constructor.It’s just a function that returns an object,a function that returns a call to function constructor.

// Define a local copy of jQuery
jQuery = function( selector, context ) {
	// The jQuery object is actually just the init constructor 'enhanced'
	// Need init if jQuery is called (just allow error to be thrown if not included)
	return new jQuery.fn.init( selector, context );
}

jQuery.fn is just an alias of jQuery.prototype which is just hanging out(just an empty object at fisrt) and is overwritten with a new object.

jQuery.extend and jQuery.fn.extend are the some function.This function takes properties and methods of one object and adds them to another.

jQuery extends itself with many useful functions,such as map,each,type,isArray,makeArray,and so on.

Sizzle

Sizzle(http://sizzlejs.com/) is a whole other engine inside of jQuery.

A pure-JavaScript CSS selector engine designed to be easily dropped in to a host library.

The jQuery.extend invocation is inside an IIFE,and the property Sizzle is returned by another IIFE that returns a Sizzle object.That is to say,one IIFE can be inside another.

jQuery is using that Sizzle library.

jQuery.find = Sizzle;
jQuery.expr = Sizzle.selectors;
jQuery.expr[":"] = jQuery.expr.pseudos;
jQuery.unique = Sizzle.uniqueSort;
jQuery.text = Sizzle.getText;
jQuery.isXMLDoc = Sizzle.isXML;
jQuery.contains = Sizzle.contains;

init and jQuery prototype

jQuery.fn.init is the function constructor.At the end of the function constructor,it’s returning a value,still giving back the empty object that was created with the new operator,but doing some stuff to it first.

init = jQuery.fn.init = function( selector, context ) {
    //elide some codes
    return jQuery.makeArray( selector, this );
}

As for any new object created with the init function constructor, its prototype is the same memory spot as jQuery.prototype–the prototype of the jQuery object itself.

//reminder: jQuery.fn = jQuery.prototype
// Give the init function the jQuery prototype for later instantiation
init.prototype = jQuery.fn;

The neat trick:

• you don’t hava to call the new operator when you use jQuery.Instead this calls a function that then calls a function constructor,creating a new object.But that new object has access to all the properties and methods in jQuery.fn.
• And instead of always in this jQuery code typing in jQuery.fn.init.something,it’s just the prototype of the jQuery object itself.

Method Chaining

method chaining: calling one method after another,and each method affects the parent object.

When you execute a method or call a method,and it’s found down the prototype chain,the this variable points at the originating object,the object that actually made the call.

e.g. Just see the return value of addClass function:

addClass: function( value ) {
    //elide some codes here
    return this;
}

Expose Globally

Expose jQuery or $ globally:

// Expose jQuery and $ identifiers, even in
// AMD (#7102#comment:10, https://github.com/jquery/jquery/pull/557)
// and CommonJS for browser emulators (#13566)
if ( typeof noGlobal === strundefined ) {
	window.jQuery = window.$ = jQuery;
}

Both of window.jQuery and window.$ are pointing at the same spot in memory as the jQuery object created inside this function that had all these properties and methods attached to it.