1、记录版本号 以及 修正constructor指向
??jquery: core_version, ??constructor: jQuery,
因为jQuery.prototype={ ... } 这种写法将自动生成的jQuery.prototype.constructor属性覆盖删除，所以需要重新修正(指向其构造函数 jQuery)。
2、init() 初始化方法：
init()方法最终返回的是this -jQuery(其实是jQuery.prototype.init）方法的实例对象。
实例对象继承了jQuery的实例属性和方法，且具有下标为0、1、2...、length的属性，从而 css()等方法可以利用 for...in循环处理 $() 集合中DOM元素。火狐下控制台截图如下：
下面分步骤分析代码：
①if：传入无效的参数selector,直接返回this。
???init: function( selector, context, rootjQuery ) { ????????????var match, elem; ????if ( !selector ) { ????????return this; ????}; ????????... ????}②if ：传入的参数selector是字符串， rquickExpr是前面声明的变量：rquickExpr = /^(?:\s*(<[\w\W]+>)[^>]*|#([\w-]*))$/；
else if ：传入的是DOM元素；
else if： 传入的是函数，调用$("document").ready(function(){ ... })；
if ：传入的是jQuery对象（存在属性selector）；
init: function( selector, context, rootjQuery ) { ???????... ????if ( typeof selector === "string" ) { ???????if ( selector.charAt(0) === "<" && selector.charAt( selector.length - 1 ) === ">" && selector.length >= 3 ) { ?match = [ null, selector, null ]; } else { ?? ?match = rquickExpr.exec( selector ); ???????}; ??????... ?????}else if ( selector.nodeType ) { this.context = this[0] = selector; this.length = 1; return this; } else if ( jQuery.isFunction( selector ) ) { return rootjQuery.ready( selector ); } if ( selector.selector !== undefined ) { this.selector = selector.selector; this.context = selector.context; } return jQuery.makeArray( selector, this ); };上面代码中，if ：情况包括类似 $("<div></div>")、$("<div>aaa</div><div>aaa</div>") 两中情况；
else ：包括其余情况，例如：$(“</div>111”)、$("#id")、$(".class")、$("div")、$("#id .class div")等情况。requickExpr匹配的是$(“</div>111”)、$("#id")两种情况。如果传入的是$(“</div>111”)，则match=["</div>111","</div>",null]；如果传入的是$("#id")，则match=["#id",null,“id”]；
③ 接下来是if esle 判断
if ：match不为null 且 match[1]不为null(传入的是标签形式)，或match不为null且content为null(传入的是id形式，无需制定上下文)；
else if： 如果context没有传入或者传入是$()对象形式，则调用$().find ；
else：如果context传入是原生jsDom对象形式,则调用$().find
if ( match && (match[1] || !context) ) { ??????}else if ( !context || context.jquery ) { ????????return ( context || rootjQuery ).find( selector );}else { ???????????????return this.constructor( context ).find( selector );}④match不为null时，又分标签形式 和 id形式
涉及到的函数：$.parseHTML 、$.merge、$.isPlainObject()
$.parseHTML ：将字符串转换为存储DOM节点的数组 。第一个参数为传入的字符串，第二个为指定的根节点，第三个是boolean值 （“<script></script>”是否可以转化为<script>标签），默认为false，不转换。
$.merge：合并两个数组，在jQuery内部不仅可以合并数组，也可以合并类数组。例如:
var ?a={0:"aaa",1:"bbb",length:2} , ????b=["ccc","ddd"];$.merge(a,b); ??//{0:"aaa",1:"bbb",2:"ccc",3:"ddd",length:4}; jQuery源码中先利用$.parseHTML 将传入的标签字符串转化为DOM节点数组，然后利用$.merge将DOM数组扩充到jQuery对象。
$.isPlainObject()：判断传入的参数是否是由 {}或new Object 创建的对象。
代码分析：
if：标签形式，（创建DOM并扩充到jQuery对象）
if ( match[1] ) { ???????//传入的第二个参数如果是jQuery对象，则转化为原生DOM context = context instanceof jQuery ? context[0] : context; //利用merge对jQuery实例对象进行扩充 jQuery.merge( this, jQuery.parseHTML( match[1], context && context.nodeType ? context.ownerDocument || context : document, true ) ); ... } else { ... }if：传入的第一个参数 是单标签字符串 且 第二个参数是纯对象 ，例如 $("<div>",{"html":"aaa","title":"aaa"});
对第二个对象进行for in 循环，如果jQuery[match]是函数，调用jQuery[match](context[match]); 否则调用jQuery.attr(match,context[match])
if ( match[1] ) { ???... ???if ( rsingleTag.test( match[1] ) && jQuery.isPlainObject( context ) ) { ?????for ( match in context ) { ????????if ( jQuery.isFunction( this[ match ] ) ) { ????????????this[ match ]( context[ match ] ); ????????} else { ???????????this.attr( match, context[ match ] ); ???????} ???} ???} ???return this;} else { ????...}else：传入的是#id 形式
if ( match[1] ) { ??...} else { ???elem = document.getElementById( match[2] ); if ( elem && elem.parentNode ) { this.length = 1; this[0] = elem; } this.context = document; this.selector = selector; return this;}相关jQuery源码：
jQuery.fn = jQuery.prototype = { ???// The current version of jQuery being used ???jquery: core_version, ????constructor: jQuery, ???init: function( selector, context, rootjQuery ) { ???????var match, elem; ????????// HANDLE: $(""), $(null), $(undefined), $(false) ???????if ( !selector ) { ???????????return this; ???????} ????????// Handle HTML strings ???????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; ????????????????????// scripts is true for back-compat ???????????????????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] ); ????????????????????// Check parentNode to catch when Blackberry 4.6 returns ???????????????????// nodes that are no longer in the document #6963 ???????????????????if ( elem && elem.parentNode ) { ???????????????????????// 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: $(DOMElement) ???????} else if ( selector.nodeType ) { ???????????this.context = this[0] = selector; ???????????this.length = 1; ???????????return this; ????????// HANDLE: $(function) ???????// Shortcut for document ready ???????} else if ( jQuery.isFunction( selector ) ) { ???????????return rootjQuery.ready( selector ); ???????} ????????if ( selector.selector !== undefined ) { ???????????this.selector = selector.selector; ???????????this.context = selector.context; ???????} ????????return jQuery.makeArray( selector, this ); ???}, ????// Start with an empty selector ???selector: "", ????// The default length of a jQuery object is 0 ???length: 0, ??????? ...};jQuery源码解析（jQuery对象的实例属性和方法）
原文地址：https://www.cnblogs.com/ZheOnaAndOnly/p/9824347.html