Totally basic Javascript reference question

Question

The following in a Javascript console:

var a = {'foo': []};
var b = {};

for (var key in a) {
   b[key] = a[key];
}

a['foo'].push(1);

console.log(b);

Yields:

Object foo=[1]

I want to make a copy by value in b of each array for each key in a. Is there an easier way?

Oscar- using Firebug inside Firefox, or Safari's Web Inspector. — Wells, Oct 23 '09 at 18:52
@Oscar: You need FireBug for `console.log` function to work. — Ivan Nevostruev, Oct 23 '09 at 18:52

Christian C. Salvadó · Answer 1 · 2009-12-22T22:06:43.283

4

You could make a "clone" function that creates a new object, based on the original object constructor, and then clone that original object properties also if they are objects:

function clone(obj){
  if(typeof(obj) != 'object' && obj != null) 
    return obj; // return the value itself if isn't an object
                // or null, since typeof  null == 'object';

    var temp = new obj.constructor();

    for(var key in obj)
        temp[key] = clone(obj[key]);
    return temp;
}


var a = {'foo': []};
var b = clone(a);

a['foo'].push(1);

console.log(b); // Object foo=[0]

edited Dec 22 '09 at 22:06

answered Oct 23 '09 at 18:34

Christian C. Salvadó

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I ran across this problem before, and this is exactly how I solved it. +1 – NateDSaint Oct 23 '09 at 18:37
1

Prototyping breaks this. Simply `Object.prototype.foo = function () {};` will create infinite recursion. – Jonathan Lonowski Oct 23 '09 at 18:41
@Jonathan: added an if statement to stop recursion. – Christian C. Salvadó Oct 23 '09 at 18:53
Same problem here CMS: `obj.constructor()` is going to die on constructor functions (`temp` will be `undefined`), and potentially clobber globals. – Crescent Fresh Dec 22 '09 at 21:27
@Crescent: Exactly, the same problem!, added the `new` operator while I think on something better... – Christian C. Salvadó Dec 22 '09 at 22:08

score 3 · Accepted Answer · edited May 23 '17 at 10:27

3

This is called Deep Copy. You can find examples in:

edited May 23 '17 at 10:27

Community

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answered Oct 23 '09 at 18:37

Ivan Nevostruev

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Anatoliy · Answer 3 · 2009-10-23T18:38:30.283

0

Easier way:

var a = {'foo': []};
var b = a;

a['foo'].push(1);

console.log(b);

Output is the same.

Edit:

var a = {'foo': []};
var b = {};

for (var key in a) {
    if (a.hasOwnProperty(key)) {
        b[key] = [];
        for (var i = 0; i < a[key].length; i += 1) {
            b[key][i] = a[key][i];
        }
    }
}

a['foo'].push(1);

console.log(b);

edited Oct 23 '09 at 18:38

answered Oct 23 '09 at 18:31

Anatoliy

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Anatoliy: I don't want b to simply be a reference to a; I want b to be a unique copy by value of the arrays stored in a so that when I push to a's arrays it does not affect b. – Wells Oct 23 '09 at 18:33
Fixed. Currently output is object with empty array. – Anatoliy Oct 23 '09 at 18:39
1

You can simplify all of that deep-copy stuff with `a[key].slice(0)`. – Eli Grey Oct 23 '09 at 19:31

score 0 · Answer 4 · answered Oct 23 '09 at 19:29

As this will add support for deep-copying arrays in your code:

var a = {'foo': []};
var b = {};

for (var key in a) {
   if (Object.prototype.toString.call(b[key]) === "[object Array]") {
      b[key] = a[key].slice(0);
   } else {
      b[key] = a[key];
   }
}

a['foo'].push(1);

console.log(b);

Totally basic Javascript reference question

4 Answers4