init的功能就是初始化jquery对象 Initialize a jQuery object

首先jQuery.fn = jQuery.prototype = {}
Query.extend = jQuery.fn.extend

// 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)
              //init是jQuery原型上面的一个函数，它也是构造函数，返回一个jQuery的实例，它经常返回this,this指代的是原型
        return new jQuery.fn.init( selector, context );
    },

init函数经常返回this.如果是作为构造函数的话，那么this是构造函数新生成的对象，继承构造函数所有this的属性，有一个自己的副本。同时， init.prototype = jQuery.fn;使得新生成的对象能够继承JQuery原型的属性。

![Uploading Paste_Image_820586.png . . .]

// Initialize a jQuery object
define( [
    "../core",
    "../var/document",
    "./var/rsingleTag",
    "../traversing/findFilter"
], function( jQuery, document, rsingleTag ) {

"use strict";

// A central reference to the root jQuery(document)
var rootjQuery,

    // A simple way to check for HTML strings
    // Prioritize #id over <tag> to avoid XSS via location.hash (#9521)
    // Strict HTML recognition (#11290: must start with <)
    // Shortcut simple #id case for speed
    rquickExpr = /^(?:\s*(<[\w\W]+>)[^>]*|#([\w-]+))$/,

       //三个参数，第一个参数是选择器
    init = jQuery.fn.init = function( selector, context, root ) {
        var match, elem;

        // HANDLE: $(""), $(null), $(undefined), $(false)
        if ( !selector ) {
            return this;
        }

        // Method init() accepts an alternate rootjQuery
        // so migrate can support jQuery.sub (gh-2101)
        root = root || rootjQuery;

        // Handle HTML strings
        if ( typeof selector === "string" ) {
            if ( selector[ 0 ] === "<" &&
                selector[ selector.length - 1 ] === ">" &&
                selector.length >= 3 ) {

                // Assume that strings that start and end with <> are HTML and skip the regex check
                match = [ null, selector, null ];

            } else {
                match = rquickExpr.exec( selector );
            }

            // Match html or make sure no context is specified for #id
            if ( match && ( match[ 1 ] || !context ) ) {

                // HANDLE: $(html) -> $(array)
                if ( match[ 1 ] ) {
                    context = context instanceof jQuery ? context[ 0 ] : context;

                    // Option to run scripts is true for back-compat
                    // Intentionally let the error be thrown if parseHTML is not present
                    jQuery.merge( this, jQuery.parseHTML(
                        match[ 1 ],
                        context && context.nodeType ? context.ownerDocument || context : document,
                        true
                    ) );

                    // HANDLE: $(html, props)
                    if ( rsingleTag.test( match[ 1 ] ) && jQuery.isPlainObject( context ) ) {
                        for ( match in context ) {

                            // Properties of context are called as methods if possible
                            if ( jQuery.isFunction( this[ match ] ) ) {
                                this[ match ]( context[ match ] );

                            // ...and otherwise set as attributes
                            } else {
                                this.attr( match, context[ match ] );
                            }
                        }
                    }

                    return this;

                // HANDLE: $(#id)
                } else {
                    elem = document.getElementById( match[ 2 ] );

                    if ( elem ) {

                        // Inject the element directly into the jQuery object
                        this[ 0 ] = elem;
                        this.length = 1;
                    }
                    return this;
                }

            // HANDLE: $(expr, $(...))
            } else if ( !context || context.jquery ) {
                return ( context || root ).find( selector );

            // HANDLE: $(expr, context)
            // (which is just equivalent to: $(context).find(expr)
            } else {
                return this.constructor( context ).find( selector );
            }

        // HANDLE: $(DOMElement)
        } else if ( selector.nodeType ) {
            this[ 0 ] = selector;
            this.length = 1;
            return this;

        // HANDLE: $(function)
        // Shortcut for document ready
        } else if ( jQuery.isFunction( selector ) ) {
            return root.ready !== undefined ?
                root.ready( selector ) :

                // Execute immediately if ready is not present
                selector( jQuery );
        }

        return jQuery.makeArray( selector, this );
    };

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

// Initialize central reference
rootjQuery = jQuery( document );

return init;

} );