jQuery对象初始化的传参方式包括：
1.$(DOMElement)
2.$('<h1>...</h1>'), $('#id'), $('.class') 传入字符串, 这是最常见的形式, 这种传参数经常也传入第二个参数context指定上下文,其中context参数可以为$(...), DOMElement
3.$(function() {}); <===> $(document).ready(function() { });
4.$({selector : '.class', context : context}) <===> $('.class', context)
jQuery.fn = jQuery.prototype = { constructor: jQuery, init: function( selector, context, rootjQuery ) { var match, elem, ret, doc; // 处理$(""), $(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 ); } };