也說Javascript對象拷貝及疑問

1、淺拷貝

當咱們須要將一個對象拷貝至另外一個對象時，咱們通常會這麼實現

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}

 淺拷貝的問題是，若是父對象的屬性等於數組或另外一個對象，實際上子對象得到的只是一個內存地址，而不是真正拷貝，父對象的數組或對象屬性發生變化時，子對象對應屬性也發生變化

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']}

2、深拷貝

爲了解決上述問題，須要對對象的數組和對象屬性進行深拷貝。它的實現並不難，只要遞歸調用"淺拷貝"就好了

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']}

上述代碼中有一個問題，當待拷貝對象中存在自引用時，程序會陷入無限循環

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

爲了解決自引用問題，拷貝時加入判斷邏輯

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'}

3、JQuery拷貝實現  

網上有不少對JQuery extend方法的分析，有不瞭解的能夠去搜索閱讀

貼一處被分析的源碼

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;
};

4、JQuery實現疑問

在閱讀上述JQuery代碼時，有個地方有疑問，疑問代碼以下

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

 

注意到，在extend方法中，爲了防止無限循環，這裏有一個邏輯，在target對象等於copy對象時，調過此次複製操做。其中，copy對象爲options對象的屬性對象。

問題是，這裏爲何要拿target對象與copy對象比較呢？難道不該該是比較copy對象和options對象嗎？

 

帶着這個疑問，在一個已經引入了JQuery庫的頁面Console中執行下

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

能夠看到，當a對象中存在自引用屬性時，extend方法並不能防止無限循環的發生

 

那麼判斷target === copy能起到什麼做用呢？

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

 

去掉判斷target === copy會陷入無限循環嗎？其實是不會的

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爲b

這裏的deepCopy是我將JQuery的extend方法，去掉上述判斷邏輯，本身實現了一份

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

 

因此這裏，是JQuery extend方法的實現bug，仍是個人理解有誤呢？

搜到的一些對JQuery extend方法的源碼分析，並無看到這個疑問，也挺奇怪的。