Push array into multidimension array in Javascript

Question

I'm facing weird situation while trying to perform simple operation of pushing array into multidimension array. Here is a code:

var Atest = Array();
var Btest = ([0, 0, 0, 0]);

Btest[0] = 1;
Btest[1] = 2
Btest[2] = 3;
Btest[3] = 4;
Atest.push([Btest]);

Btest[0] = 11;
Btest[1] = 12;
Btest[2] = 13;
Btest[3] = 14;
Atest.push([Btest]);

document.write("<br>" + Atest);

And I'm expecting to get the following output:

1,2,3,4,11,12,13,14

However, I'm getting unexpected output:

11,12,13,14,11,12,13,14

What I'm missing?

(PS: Found similar unanswered question asked ~5 years ago: pushing new array into a 2d array)

This is because arrays are mutable, so you need to deep clone while pushing to a new array — Rahul Sharma
– Rahul Sharma, Commented Jan 28, 2021 at 10:59

Orelsanpls · Accepted Answer · 2021-01-28 10:57:06Z

2

When you push Btest into Atest you push a pointer to the Btest array.

You need to copy the underlying values contained inside of Btest.

Example using the spread operator which will create a copy of the data :

const Atest = Array();
const Btest = ([0, 0, 0, 0]);

Btest[0] = 1;
Btest[1] = 2
Btest[2] = 3;
Btest[3] = 4;
Atest.push([...Btest]);

Btest[0] = 11;
Btest[1] = 12;
Btest[2] = 13;
Btest[3] = 14;
Atest.push([...Btest]);

document.write(`<br>${Atest}`);

edited Jan 28, 2021 at 10:57

answered Jan 28, 2021 at 10:55

Orelsanpls

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2 Comments

evolutionxbox Over a year ago

I don't think JS uses pointers (although I think this is an engine implementation detail)

evolutionxbox Over a year ago

no worries. I personally don't think it matters much, but I hope it helps

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