也说Javascript对象拷贝及疑问

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也说Javascript对象拷贝及疑问

来源: 作者: 时间:2016-02-01 09:32 【

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functi
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

复制代码
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
 
 
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);

 


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