Copying an Object in Javascript

Question

I'd like a simple, straight-forward answer to this question. What better place on the web than to archive it here?

I have an object x. I'd like to copy it as object y, such that changes to y do not modify x.

What's the most elegant way of doing this in javascript?

Edit: I realize that copying objects derived from built in javascript objects will result in extra, unwanted properties. This isn't a problem, since I'm copying one of my own, literal-constructed objects.

Answer 1

Recursive for in loop is what you will have to work with.

Answer 2

From this article: How to copy arrays and objects in Javascript by Brian Huisman:

Object.prototype.clone = function() {
  var newObj = (this instanceof Array) ? [] : {};
  for (i in this) {
    if (i == 'clone') continue;
    if (this[i] && typeof this[i] == "object") {
      newObj[i] = this[i].clone();
    } else newObj[i] = this[i]
  } return newObj;
};

Answer 3

Here is a function you can use.

function clone(obj){

    if(obj == null || typeof(obj) != 'object')

        return obj;



    var temp = new obj.constructor(); 

    for(var key in obj)

        temp[key] = clone(obj[key]);



    return temp;

}

Answer 4

If there are no circular dependencies in your object, I suggest using one of the other answers or jQuery's copy methods, as they all seem quite effective.

If there are circular dependencies (i.e., two sub-objects link to each other), you are kind of screwed as there is (from a theoretical perspective) no way to solve this issue elegantly.

Answer 5

Using Prototype framework:

var y = Object.clone(x);

Considering clone implementation:

...
Object.extend = function(destination, source) {
  for (var property in source)
    destination[property] = source[property];
  return destination;
};
...
clone: function(object) {
    return Object.extend({ }, object);
  }
...

Answer 6

From the Apple JavaScript Coding Guidelines:

// Create an inner object with a variable x whose default
// value is 3.
function innerObj()
{
        this.x = 3;
}
innerObj.prototype.clone = function() {
    var temp = new innerObj();
    for (myvar in this) {
        // this object does not contain any objects, so
        // use the lightweight copy code.
        temp[myvar] = this[myvar];
    }
    return temp;
}

// Create an outer object with a variable y whose default
// value is 77.
function outerObj()
{
        // The outer object contains an inner object.  Allocate it here.
        this.inner = new innerObj();
        this.y = 77;
}
outerObj.prototype.clone = function() {
    var temp = new outerObj();
    for (myvar in this) {
        if (this[myvar].clone) {
            // This variable contains an object with a
            // clone operator.  Call it to create a copy.
            temp[myvar] = this[myvar].clone();
        } else {
            // This variable contains a scalar value,
            // a string value, or an object with no
            // clone function.  Assign it directly.
            temp[myvar] = this[myvar];
        }
    }
    return temp;
}

// Allocate an outer object and assign non-default values to variables in
// both the outer and inner objects.
outer = new outerObj;
outer.inner.x = 4;
outer.y = 16;

// Clone the outer object (which, in turn, clones the inner object).
newouter = outer.clone();

// Verify that both values were copied.
alert('inner x is '+newouter.inner.x); // prints 4
alert('y is '+newouter.y); // prints 16

Steve

Answer 7

To do this for any object in JavaScript will not be simple or straightforward. You will run into the problem of erroneously picking up attributes from the object's prototype that should be left in the prototype and not copied to the new instance. If, for instance, you are adding a clone method to Object.prototype, as some answers depict, you will need to explicitly skip that attribute. But what if there are other additional methods added to Object.prototype, or other intermediate prototypes, that you don't know about? In that case, you will copy attributes you shouldn't, so you need to detect unforeseen, non-local attributes with the hasOwnProperty method.

In addition to non-enumerable attributes, you'll encounter a tougher problem when you try to copy objects that have hidden properties. For example, prototype is a hidden property of a function. Also, an object's prototype is referenced with the attribute __proto__, which is also hidden, and will not be copied by a for/in loop iterating over the source object's attributes. I think __proto__ might be specific to Firefox's JavaScript interpreter and it may be something different in other browsers, but you get the picture. Not everything is enumerable. You can copy a hidden attribute if you know its name, but I don't know of any way to discover it automatically.

Yet another snag in the quest for an elegant solution is the problem of setting up the prototype inheritance correctly. If your source object's prototype is Object, then simply creating a new general object with {} will work, but if the source's prototype is some descendant of Object, then you are going to be missing the additional members from that prototype which you skipped using the hasOwnProperty filter, or which were in the prototype, but weren't enumerable in the first place. One solution might be to call the source object's constructor property to get the initial copy object and then copy over the attributes, but then you still will not get non-enumerable attributes. For example, a Date object stores its data as a hidden member:

function clone(obj) {
    if (null == obj || "object" != typeof obj) return obj;
    var copy = obj.constructor();
    for (var attr in obj) {
        if (obj.hasOwnProperty(attr)) copy[attr] = obj[attr];
    }
    return copy;
}

var d1 = new Date();

/* Wait for 5 seconds. */
var start = (new Date()).getTime();
while (new Date()).getTime() - start < 5000);


var d2 = clone(d1);
alert("d1 = " + d1.toString() + "\nd2 = " + d2.toString());

The date string for d1 will be 5 seconds behind that of d2. A way to make one Date the same as another is by calling the setTime method, but that is specific to the Date class. I don't think there is a bullet-proof general solution to this problem, though I would be happy to be wrong!

When I had to implement general deep copying I ended up compromising by assuming that I would only need to copy a plain Object, Array, Date, String, Number, or Boolean. The last 3 types are immutable, so I could perform a shallow copy and not worry about it changing. I further assumed that any elements contained in Object or Array would also be one of the 6 simple types in that list. This can be accomplished with code like the following:

function clone(obj) {
    // Handle the 3 simple types, and null or undefined
    if (null == obj || "object" != typeof obj) return obj;

    // Handle Date
    if (obj instanceof Date) {
        var copy = new Date();
        copy.setTime(obj.getTime());
        return copy;
    }

    // Handle Array
    if (obj instanceof Array) {
        var copy = [];
        for (var i = 0, var len = obj.length; i < len; ++i) {
            copy[i] = clone(obj[i]);
        }
        return copy;
    }

    // Handle Object
    if (obj instanceof Object) {
        var copy = {};
        for (var attr in obj) {
            if (obj.hasOwnProperty(attr)) copy[attr] = clone(obj[attr]);
        }
        return copy;
    }

    throw new Error("Unable to copy obj! Its type isn't supported.");
}

The above function will work adequately for the 6 simple types I mentioned, as long as the data in the objects and arrays form a tree structure. That is, there isn't more than one reference to the same data in the object. For example:

// This would be cloneable:
var tree = {
    "left"  : { "left" : null, "right" : null, "data" : 3 },
    "right" : null,
    "data"  : 8
};

// This would kind-of work, but you would get 2 copies of the 
// inner node instead of 2 references to the same copy
var directedAcylicGraph = {
    "left"  : { "left" : null, "right" : null, "data" : 3 },
    "data"  : 8
};
directedAcyclicGraph["right"] = directedAcyclicGraph["left"];

// Cloning this would cause a stack overflow due to infinite recursion:
var cylicGraph = {
    "left"  : { "left" : null, "right" : null, "data" : 3 },
    "data"  : 8
};
cylicGraph["right"] = cylicGraph;

It will not be able to handle any JavaScript object, but it may be sufficient for many purposes as long as you don't assume that it will just work for anything you throw at it.

Answer 8

One particular "un-elegant" solution is to use JSON encoding to make deep copies of objects that do not have member methods. The methodology is to JSON encode your target object, then by decoding it, you get the copy you are looking for. You can decode as many times as you want to make as many copies as you need.

Of course, functions do not belong in JSON, so this only works for objects without member methods.

This methodology was perfect for my use case, since I'm storing JSON blobs in a key-value store, and when they are exposed as objects in a JavaScript API, each object actually contains a copy of the original state of the object so we can calculate the delta after the caller has mutated the exposed object.