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);

View Code

 

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。