Clone Object without reference javascript [duplicate]

I have a big object with much data. And i want to clone this in other variable. When i set some param of the instance B has the same result in the original object:

var obj = {a: 25, b: 50, c: 75};
var A = obj;
var B = obj;

A.a = 30;
B.a = 40;

alert(obj.a + " " + A.a + " " + B.a); // 40 40 40

My output should be 25 30 40. Any ideas?

EDIT

Thanks Everyone. I change the code of dystroy and this is my result:

Object.prototype.clone = Array.prototype.clone = function()
{
    if (Object.prototype.toString.call(this) === '[object Array]')
    {
        var clone = [];
        for (var i=0; i<this.length; i++)
            clone[i] = this[i].clone();

        return clone;
    } 
    else if (typeof(this)=="object")
    {
        var clone = {};
        for (var prop in this)
            if (this.hasOwnProperty(prop))
                clone[prop] = this[prop].clone();

        return clone;
    }
    else
        return this;
}

var obj = {a: 25, b: 50, c: 75};
var A = obj.clone();
var B = obj.clone();
A.a = 30;
B.a = 40;
alert(obj.a + " " + A.a + " " + B.a);

var arr = [25, 50, 75];
var C = arr.clone();
var D = arr.clone();
C[0] = 30;
D[0] = 40;
alert(arr[0] + " " + C[0] + " " + D[0]);

Solution 1:

If you use an = statement to assign a value to a var with an object on the right side, javascript will not copy but reference the object.

Spoiler : using JSON.parse(JSON.stringify(obj)) may work but is costly, and might throw a TypeError as in

const a = {};
const b = { a };
a.b = b;
const clone = JSON.parse(JSON.stringify(a));
/* Throws
Uncaught TypeError: Converting circular structure to JSON
    --> starting at object with constructor 'Object'
    |     property 'b' -> object with constructor 'Object'
    --- property 'a' closes the circle
    at JSON.stringify (<anonymous>)
    at <anonymous>:4:6
*/

As of es2015, if you want a shallow copy (clone the object, but keeping deep refences in the inner structure) you can use destructuring :

const obj = { foo: { bar: "baz" } };
const shallowClone = { ...obj };

shallowClone is a new object, but shallowClone.foo holds a reference to the same object as obj.foo.

You can use lodash's clone method, which does the same, if you don't have access to the spread operator.

var obj = {a: 25, b: 50, c: 75};
var A = _.clone(obj);

Or lodash's cloneDeep method if your object has multiple object levels

var obj = {a: 25, b: {a: 1, b: 2}, c: 75};
var A = _.cloneDeep(obj);

Or lodash's merge method if you mean to extend the source object

var obj = {a: 25, b: {a: 1, b: 2}, c: 75};
var A = _.merge({}, obj, {newkey: "newvalue"});

Or you can use jQuerys extend method:

var obj = {a: 25, b: 50, c: 75};
var A = $.extend(true,{},obj);

Here is jQuery 1.11 extend method's source code :

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;

        // skip the boolean and the target
        target = arguments[ i ] || {};
        i++;
    }

    // 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 ( i === length ) {
        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;
};

var item ={ 'a': 1, 'b': 2}
Object.assign({}, item);

Solution 2:

While this isn't cloning, one simple way to get your result is to use the original object as the prototype of a new one.

You can do this using Object.create:

var obj = {a: 25, b: 50, c: 75};
var A = Object.create(obj);
var B = Object.create(obj);

A.a = 30;
B.a = 40;

alert(obj.a + " " + A.a + " " + B.a); // 25 30 40

This creates a new object in A and B that inherits from obj. This means that you can add properties without affecting the original.

To support legacy implementations, you can create a (partial) shim that will work for this simple task.

if (!Object.create)
    Object.create = function(proto) {
        function F(){}
        F.prototype = proto;
        return new F;
    }

It doesn't emulate all the functionality of Object.create, but it'll fit your needs here.

Solution 3:

You could define a clone function.

I use this one :

function goclone(source) {
    if (Object.prototype.toString.call(source) === '[object Array]') {
        var clone = [];
        for (var i=0; i<source.length; i++) {
            clone[i] = goclone(source[i]);
        }
        return clone;
    } else if (typeof(source)=="object") {
        var clone = {};
        for (var prop in source) {
            if (source.hasOwnProperty(prop)) {
                clone[prop] = goclone(source[prop]);
            }
        }
        return clone;
    } else {
        return source;
    }
}

var B = goclone(A);

It doesn't copy the prototype, functions, and so on. But you should adapt it (and maybe simplify it) for you own need.