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)

• evolutionxbox: The values of Btest are being modified twice.
• Najam Us Saqib: you should use atest.concat(btest)
• Rahul Sharma: This is because arrays are mutable, so you need to deep clone while pushing to a new array

Answer 1

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}`);

• evolutionxbox: I don't think JS uses pointers (although I think this is an engine implementation detail)
• evolutionxbox: no worries. I personally don't think it matters much, but I hope it helps