也说Javascript对象拷贝及疑问

一、浅拷贝

当我们需要将一个对象拷贝至另一个对象时，我们一般会这么实现

function shadowCopy(source,target){
    var target=target||{};
    for(var i in source)
    {
        target[i]=source[i];
    }
    return target;
}

var a={name:'Lily',age:19};
var b=shadowCopy(a);//b={name:'Lily',age:19}

浅拷贝的问题是，如果父对象的属性等于数组或另一个对象，实际上子对象获得的只是一个内存地址，而不是真正拷贝，父对象的数组或对象属性发生变化时，子对象对应属性也发生变化

function shadowCopy(source,target){
    var target=target||{};
    for(var i in source)
    {
        target[i]=source[i];
    }
    return target;
}

var a={name:'Lily',Hobbies:['Music','Sport']};
var b=shadowCopy(a);//b={name:'Lily',Hobbies:['Music','Sport']}
a.Hobbies.push('Read');//b={name:'Lily',Hobbies:['Music','Sport','Read']}

二、深拷贝

为了解决上述问题，需要对对象的数组和对象属性进行深拷贝。它的实现并不难，只要递归调用"浅拷贝"就行了

function deepCopy(source,target){
    var target=target||{};
    for(var i in source)
    {
        if(typeof source[i] === 'object'){
            target[i] = (source[i].constructor === Array ) ? [] : {} ;
            deepCopy(source[i],target[i]);
        }else{
            target[i]=source[i];
        }
    }
    return target;
}
var a={name:'Lily',Hobbies:['Music','Sport']};
var b=deepCopy(a);//b={name:'Lily',Hobbies:['Music','Sport']}
a.Hobbies.push('Read');//b={name:'Lily',Hobbies:['Music','Sport','Read']},b={name:'Lily',Hobbies:['Music','Sport']}

上述代码中有一个问题，当待拷贝对象中存在自引用时，程序会陷入无限循环

var a={name:'lily'};
a.obj=a;
deepCopy(a);
在Chome Console运行时，如下提示
RangeError: Maximum call stack size exceeded

为了解决自引用问题，拷贝时加入判断逻辑

function deepCopy(source,target){
    var target=target||{};
    for(var i in source)
    {
                //防止自引用
        if(source[i] === source )
            continue;
        if(typeof source[i] === 'object'){
            target[i] = (source[i].constructor === Array ) ? [] : {} ;
            deepCopy(source[i],target[i]);
        }else{
            target[i]=source[i];
        }
    }
    return target;
}             

var a={name:'lily'};
a.obj=a;
var b=deepCopy(a);//b={name:'lily'}

三、JQuery拷贝实现

网上有很多对JQuery extend方法的分析，有不了解的可以去搜索阅读

贴一处被分析的源码

jQuery.extend = jQuery.fn.extend = function() {
    var src, copyIsArray, copy, name, options, 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;
};

四、JQuery实现疑问

在阅读上述JQuery代码时，有个地方有疑问，疑问代码如下

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

注意到，在extend方法中，为了防止无限循环，这里有一个逻辑，在target对象等于copy对象时，调过这次复制操作。其中，copy对象为options对象的属性对象。

问题是，这里为什么要拿target对象与copy对象比较呢？难道不应该是比较copy对象和options对象吗？

带着这个疑问，在一个已经引入了JQuery库的页面Console中执行下

var a={name:'lily'};
a.obj=a;
var b={};
$.extend(true,b,a);
RangeError: Maximum call stack size exceeded

可以看到，当a对象中存在自引用属性时，extend方法并不能防止无限循环的发生

那么判断target === copy能起到什么作用呢？

var a={name:'lily'};
var b={age:19};
a.obj=b;
$.extend(true,b,a);
//此时b={age: 19, name: "lily"}

去掉判断target === copy会陷入无限循环吗？实际上是不会的

var a={name:'lili'};
var b={age:19};
a.obj=b;
deepCopy(true,b,a);
//b=Object {age: 19, name: "lili", obj: Object}
//其中Object为b

这里的deepCopy是我将JQuery的extend方法，去掉上述判断逻辑，自己实现了一份

function deepCopy() {
    var src, copyIsArray, copy, name, options, 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 ( options === copy ) {
                    continue;
                }
                */
                // Recurse if we're merging plain objects or arrays
                if ( deep && copy && ( isPlainObject(copy) || (copyIsArray =isArray(copy)) ) ) {
                    if ( copyIsArray ) {
                        copyIsArray = false;
                        clone = src && isArray(src) ? src : [];

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

                    // Never move original objects, clone them
                    target[ name ] =deepCopy( deep, clone, copy );

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

    // Return the modified object
    return target;
};

var isString=function(obj){
    return Object.prototype.toString.call(obj) === '[object String]';
};
var isArray=function(obj){
    return Object.prototype.toString.call(obj) === '[object Array]';
};
var isPlainObject=function(obj){
    return Object.prototype.toString.call(obj) === '[object Object]';
}
var a={name:'lili'};
var b={age:19};
a.obj=b;
deepCopy(true,b,a);

所以这里，是JQuery extend方法的实现bug，还是我的理解有误呢？

搜到的一些对JQuery extend方法的源码分析，并没有看到这个疑问，也挺奇怪的。