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title: 解读jQuery中extend函数 categories:

• JavaScript
• 杂物间 tags:
• tech
• cnblogs warning: true date: 2013-07-20 01:59:00 fromUrl: http://www.cnblogs.com/hustskyking/archive/2013/07/20/extend-in-jQuery.html from: 博客园

$.extend.apply( null, [ true, { "a" : 1, "b" : 2 } ] );//console.log(window.a);//window.location.reload();
$.extend.apply( null, [ true, { "a" : 1, "b" : 2 } ].concat( { "c" : 3, "d" : 4 } ) );
//console.log(window.a)

jQuery.extend = jQuery.fn.extend = function() {
    // copy reference to target object
    var target = arguments[0] || {}, i = 1, length = arguments.length, deep = false, options;

    // Handle a deep copy situation
    if ( target.constructor == Boolean ) {
        deep = target;
        target = arguments[1] || {};
        // skip the boolean and the target
        i = 2;
    }

    // Handle case when target is a string or something (possible in deep copy)
    if ( typeof target != "object" && typeof target != "function" )
        target = {};

    // extend jQuery itself if only one argument is passed
    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 ( var name in options ) {
                var src = target[ name ], copy = options[ name ];

                // Prevent never-ending loop
                if ( target === copy )
                    continue;

                // Recurse if we're merging object values
                if ( deep && copy && typeof copy == "object" && !copy.nodeType )
                    target[ name ] = jQuery.extend( deep,
                        // Never move original objects, clone them
                        src || ( copy.length != null ? [ ] : { } )
                    , copy );

                // Don't bring in undefined values
                else if ( copy !== undefined )
                    target[ name ] = copy;

            }

    // Return the modified object
    return target;
};

jQuery - v1.2.6 extend

jQuery.extend = jQuery.fn.extend = function() {
    var options, name, src, copy, copyIsArray, clone,
        target = arguments[0] || {},
        i = 1,
        length = arguments.length,
        deep = false;

    // Handle a deep copy situation
    if ( typeof target === "boolean" ) {
        deep = target;
        target = arguments[1] || {};
        // skip the boolean and the target
        i = 2;
    }

    // Handle case when target is a string or something (possible in deep copy)
    if ( typeof target !== "object" && !jQuery.isFunction(target) ) {
        target = {};
    }

    // extend jQuery itself if only one argument is passed
    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;
};

jQuery - V1.8.2 extend

　　第一个参数是boolean类型 ---------> 深度递归复制
               |
               |
               ---------> 如果只有一个参数（除第一个boolean）---------> 将函数的this环境extend进去
                                       |
                                       |
                                       ---------> 将后面的参数extend到\第一个"参数中 ---------> 返回\第一个"参数

window 
<p><span>即，window.a = 1</span></p>
<p>&nbsp;<img src="https://images.cnitblog.com/blog/387325/201307/20134313-96862ff8cfa84a72a7ef6ab81b6d66a6.jpg" alt=""></p>


<ul>
<li><strong><span>第二个问题中，</span></strong></li>
</ul>

$.extend.apply( null, [ true, { "a" : 1, "b" : 2 } ].concat( { "c" : 3, "d" : 4 } ) );

<p>相当于传入三个参数，第一个是true，第二个是 匿名对象{ "a" : 1, "b" : 2 }，第三个是匿名对象{ "c" : 3, "d" : 4 }, 此时的this环境是null（因为将函数绑定到null上面了）。</p>
<p>那么结果就是{ "a" : 1, "b" : 2,&nbsp;"c" : 3, "d" : 4 }, 此刻并不能在window中索引到a这个属性</p>
<p><img src="https://images.cnitblog.com/blog/387325/201307/20134220-8b0da1f1b68c41d190e65c9925dca2c4.jpg" alt=""></p>
<p>如要想要访问，那便是null.a，但这种访问方式肯定是不对的。</p>
<p>由于没有变量来接收这个返回的值，所以被作为垃圾给回收了。</p>


<p>有兴趣的童鞋可以试试这个：</p>

$.extend.apply( $, [ true, { "a" : 1, "b" : 2 } ] ); $.extend.apply( $, [ true, { "a" : 1, "b" : 2 } ].concat( { "c" : 3, "d" : 4 } ) );

```