CODEWITHSUNDEEP
javascriptdeep-copy
Wells
Wells

Reputation: 10979

Totally basic Javascript reference 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?

Upvotes: 8

Views: 360

Answers (4)

Christian C. Salvadó
Christian C. Salvadó

Reputation: 828060

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]

Upvotes: 4

Eli Grey
Eli Grey

Reputation: 35913

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);

Upvotes: 0

Anatoliy
Anatoliy

Reputation: 30103

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);

Upvotes: 0

Ivan Nevostruev
Ivan Nevostruev

Reputation: 28753

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

Upvotes: 3

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