Is there a copy constructor in Javascript?

Question

I am creating an object in javascript. However, I know that I am using the new className() operator. I want to know if I can make a copy constructor so when I create a new object ie var object1 = object2, only the member variables in object2 are copied into object1, and not the pointer. Thanks!

Related: [Most elegant way to clone a JavaScript object](http://stackoverflow.com/questions/728360/most-elegant-way-to-clone-a-javascript-object) / [What is the most efficient way to clone an object?](http://stackoverflow.com/questions/122102/what-is-the-most-efficient-way-to-clone-an-object) — Jonathan Lonowski, May 08 '15 at 15:10
In the manner you described, no. For other options, use the search, this question has many duplicates. — Etheryte, May 08 '15 at 15:10

ssube · Accepted Answer · 2015-05-13T16:53:32.360

JS does not automatically generate constructors -- copy, move, or otherwise -- for any objects. You have to define them yourself.

The closest you'll come is something like Object.create, which takes a prototype and an existing object to copy the properties.

To define a copy constructor, you can start with something along the lines of:

function Foo(other) {
  if (other instanceof Foo) {
    this.bar = other.bar;
  } else {
    this.bar = other;
  }
}

var a = new Foo(3);
var b = new Foo(a);

document.getElementById('bar').textContent = b.bar;

<pre id="bar"></pre>

Using this to support deep copying is just a recursion of the same pattern:

function Foo(other) {
  if (other instanceof Foo) {
    this.bar = new Bar(other.bar);
  } else {
    this.bar = new Bar(other);
  }
}

function Bar(other) {
  if (other instanceof Bar) {
    this.val = other.val;
  } else {
    this.val = other;
  }
}

Bar.prototype.increment = function () {
  this.val++;
}

Bar.prototype.fetch = function () {
  return this.val;
}

var a = new Foo(3);
var b = new Foo(a);
a.bar.increment();

document.getElementById('a').textContent = a.bar.fetch();
document.getElementById('b').textContent = b.bar.fetch();

<pre id="a"></pre>
<pre id="b"></pre>

Is there a copy constructor in Javascript?

1 Answers1