Javascript: create a copy of an array using slice();

Question

I need to create a copy of an array so that I can do modification on one array without affecting the other.

for example:

var a = [[1],[2],[3]];
var b = a.slice(0);
b[1].push(100);
b // [[1],[2, 100],[3]];
a // expect: [[1],[2],[3]]; , actual: [[1],[2, 100],[3]];

I have also tried:

var b = new Array(a);

but this puts all [1],[2],[3] to index 0 in the new array.

What am I missing here?

Thanks guys!

use the JSON trick: b = JSON.parse(JSON.stringify(a)) this will do some kind of a "deep copy", then you can modify b without messing a. — Calvin Nunes
– Calvin Nunes, Commented Aug 31, 2018 at 18:35

Jonas Wilms · Accepted Answer · 2018-08-31 18:31:28Z

2

you also have to copy the inner arrays:

 var b = a.map(sub => sub.slice());

answered Aug 31, 2018 at 18:31

Jonas Wilms

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Comments

Emeeus · Accepted Answer · 2018-08-31 18:44:06Z

0

slice() makes a shallow copy, for n levels you could use JSON.parse(JSON.stringify(a)) or recursion:

var a = [[[1]],[2],[3]];

var b = JSON.parse(JSON.stringify(a))

b[0][0][0] = 999;

console.log(a)

console.log(b)

answered Aug 31, 2018 at 18:44

Emeeus

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