jQuery.extend源码深层分析

2022-12-31 14:50:32

　　在网站的开发中，经常会自己写一些jQuery插件来方便使用，其中自然少不了一个关键的方法->jQuery.extend()，使用这个方法来扩展jQuery对象。

那么今天就来讲讲这个函数的实现原理。

　　这个方法不仅仅可以实现插件，当然也有它本来的功能，扩展对象。like this:

var person = {name:'李四'};

var person2 = $.extend(person,{age:20},{sex:'male'});

//person.name='李四';

//person.age=20;

//person.sex='male';
person2与person扩展后是同一个对象

这个函数的第一个函数指的是要被扩充的对象(target)，第二、第三个函数则是附加对象。通俗点将，jQuery将检查target中没有的属性，而在后面两个对象中却存在，那么这些属性将被添加到target中。

那么我们写插件又是怎么回事呢，我们只用了一个参数啊，像这样：

$.extend({

    myExtendMethod:function(){

        alert('这是我的方法');

    }

});

像上面这个例子里，我们的$.extend方法中只有一个Object类型的参数，即：{myExtendMethod}，那么这个时候怎么扩展，去扩展谁？target又是什么？当然要扩展的就是我们爱不释手的jQuery了！上一下源码：

 jQuery.extend = jQuery.fn.extend = function() {

     var options, name, src, copy, copyIsArray, clone,

         target = arguments[0] || {},

         i = 1,

         length = arguments.length,

         deep = false;

     // 不会满足，因为我们传进来的是Object

     if ( typeof target === "boolean" ) {

         deep = target;

         target = arguments[1] || {};

         // skip the boolean and the target

         i = 2;

     }

     // 同样不会满足

     if ( typeof target !== "object" && !jQuery.isFunction(target) ) {

         target = {};

     }

     // length为1，i也为1，所以会走这个条件

     if ( length === i ) {

         target = this;

         --i;

     }

     for ( ; i < length; i++ ) {

         // Only deal with non-null/undefined values

         if ( (options = arguments[ i ]) != null ) {

             // Extend the base object

             for ( name in options ) {

                 src = target[ name ];

                 copy = options[ name ];

                 // Prevent never-ending loop

                 if ( target === copy ) {

                     continue;

                 }

                 // Recurse if we're merging plain objects or arrays

                 if ( deep && copy && ( jQuery.isPlainObject(copy) || (copyIsArray = jQuery.isArray(copy)) ) ) {

                     if ( copyIsArray ) {

                         copyIsArray = false;

                         clone = src && jQuery.isArray(src) ? src : [];

                     } else {

                         clone = src && jQuery.isPlainObject(src) ? src : {};

                     }

                     // Never move original objects, clone them

                     target[ name ] = jQuery.extend( deep, clone, copy );

                 // Don't bring in undefined values

                 } else if ( copy !== undefined ) {

                     target[ name ] = copy;

                 }

             }

         }

     }

     // Return the modified object

     return target;

 };

通过这段代码可以看到第一行的定义，$.extend和$.fn.extend的定义方法为同一个匿名函数，也就是他们是一个方法实现的，只是扩展的对象前者是jQuery，

后者是jQuery.fn =jQuery.prototype也就是 jQuery实例（ $('div') ）。$.fn.extend()就是为jQuery添加实例方法。

ps：如果不了解js原型概念可以看我上一篇文章js创建对象，或者看其他园子的文章。

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" alt="" /> 源码：jQuery.fn= jQuery.prototype

下面就来看看$.extend函数只有一个参数时怎么执行的。

看3行代码的target变量，指定了arguments[0]为它的值，按照刚刚我们定义的插件，就是{myExtendMethod:function(){...}}这个object;

继续往下看，看我21行的注释，this指的是jQuery对象，然后把jQuery赋给了target变量，并且把 i 变成了0 ；

然后进入for循环，在第55行代码的时候，会把我们的参数Object赋给jQuery对象，这样，就把myExtendMethod这个方法添加到jQuery对象上了。

 $.extend({

     testMethod: function () {

          alert(111);

     }

 });

 $.fn.extend({

     testMethod2: function () {

          alert(222);

     }

  });

 //调用扩展方法

 $.testMethod();//弹出111

 $("#List1").testMethod2();//弹出222

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