Nodejs: how to clone an object

Question

If I clone an array, I use cloneArr = arr.slice()

I want to know how to clone an object in nodejs.

Answer 1

For a shallow copy, I like to use the reduce pattern (usually in a module or such), like so:

var newObject = Object.keys(original).reduce(function (obj, item) {
    obj[item] = original[item];
    return obj;
},{});

Here's a jsperf for a couple of the options: http://jsperf.com/shallow-copying

Answer 2

There are some Node modules out there if don't want to "roll your own". This one looks good: https://www.npmjs.com/package/clone

Looks like it handles all kinds of stuff, including circular references. From the github page:

clone masters cloning objects, arrays, Date objects, and RegEx objects. Everything is cloned recursively, so that you can clone dates in arrays in objects, for example. [...] Circular references? Yep!

Answer 3

Depending on what you want to do with your cloned object you can utilize the prototypal inheritence mechanism of javascript and achieve a somewhat cloned object through:

var clonedObject = Object.create(originalObject);

Just remember that this isn't a full clone - for better or worse.

A good thing about that is that you actually haven't duplicated the object so the memory footprint will be low.

Some tricky things to remember though about this method is that iteration of properties defined in the prototype chain sometimes works a bit different and the fact that any changes to the original object will affect the cloned object as well unless that property has been set on itself also.

Answer 4

for array, one can use

var arr = [1,2,3]; 
var arr_2 = arr ; 

print ( arr_2 ); 

arr=arr.slice(0);

print ( arr ); 

arr[1]=9999; 

print ( arr_2 ); 

Answer 5

Object.assign hasn't been mentioned in any of above answers.

let cloned = Object.assign({}, source);

If you're on ES6 you can use the spread operator:

let cloned = { ... source };

Reference: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign

Answer 6

It's hard to do a generic but useful clone operation because what should be cloned recursively and what should be just copied depends on how the specific object is supposed to work.

Something that may be useful is

function clone(x)
{
    if (x === null || x === undefined)
        return x;
    if (typeof x.clone === "function")
        return x.clone();
    if (x.constructor == Array)
    {
        var r = [];
        for (var i=0,n=x.length; i<n; i++)
            r.push(clone(x[i]));
        return r;
    }
    return x;
}

In this code the logic is

• in case of null or undefined just return the same (the special case is needed because it's an error to try to see if a clone method is present)
• does the object have a clone method ? then use that
• is the object an array ? then do a recursive cloning operation
• otherwise just return the same value

This clone function should allow implementing custom clone methods easily... for example

function Point(x, y)
{
    this.x = x;
    this.y = y;

    ...
}

Point.prototype.clone = function()
{
    return new Point(this.x, this.y);
};



function Polygon(points, style)
{
    this.points = points;
    this.style = style;

    ...
}

Polygon.prototype.clone = function()
{
    return new Polygon(clone(this.points),
                       this.style);
};

When in the object you know that a correct cloning operation for a specific array is just a shallow copy then you can call values.slice() instead of clone(values).

For example in the above code I am explicitly requiring that a cloning of a polygon object will clone the points, but will share the same style object. If I want to clone the style object too instead then I can just pass clone(this.style).