jQuery.fn = jQuery.prototype = {
    constructor: jQuery,
    init: function( selector, context, rootjQuery ) {
        var match, elem, ret, doc;
        // 处理$(""), $(null), $(undefined), $(false)这几种参数,直接返回this
        if ( !selector ) {
            return this;
        }
        // 当传参selector为DOM结点时,将context置为selector
        if ( selector.nodeType ) {
            this.context = this[0] = selector;
            this.length = 1;
            return this;
        }
        // Handle HTML strings
        // 当传入的selector参数为字符串时,
        if ( typeof selector === "string" ) {
            if ( selector.charAt(0) === "<" && selector.charAt( 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;
                    doc = ( context && context.nodeType ? context.ownerDocument || context : document );
                    // scripts is true for back-compat
                    selector = jQuery.parseHTML( match[1], doc, true );
                    if ( rsingleTag.test( match[1] ) && jQuery.isPlainObject( context ) ) {
                        this.attr.call( selector, context, true );
                    }
                    return jQuery.merge( this, selector );
                // HANDLE: $(#id)
                } else {
                    elem = document.getElementById( match[2] );
                    // Check parentNode to catch when Blackberry 4.6 returns
                    // nodes that are no longer in the document #6963
                    if ( elem && elem.parentNode ) {
                        // Handle the case where IE and Opera return items
                        // by name instead of ID
                        if ( elem.id !== match[2] ) {
                            return rootjQuery.find( selector );
                        }
                        // Otherwise, we inject the element directly into the jQuery object
                        this.length = 1;
                        this[0] = elem;
                    }
                    this.context = document;
                    this.selector = selector;
                    return this;
                }
            // HANDLE: $(expr, $(...))
            } else if ( !context || context.jquery ) {
                return ( context || rootjQuery ).find( selector );
            // HANDLE: $(expr, context)
            // (which is just equivalent to: $(context).find(expr)
            } else {
                return this.constructor( context ).find( selector );
            }
        // HANDLE: $(function)
        // Shortcut for document ready
        // 当selector为function时相当于$(document).ready(selector);
        } else if ( jQuery.isFunction( selector ) ) {
            return rootjQuery.ready( selector );
        }
        // 当selector参数为{selector:'#id', context:document}之类时,重置属性selector和context
        if ( selector.selector !== undefined ) {
            this.selector = selector.selector;
            this.context = selector.context;
        }
        return jQuery.makeArray( selector, this );
    }
};