How to transpose 2D square matrix stored as 1D array in Javascript

Question

My question is closely related to this question but I'm looking for a solution in Javascript

How to Transpose 2D Matrix Stored as C 1D Array

Basically I have a 2D square matrix

1 2 3
4 5 6
7 8 9

Stored as follows

let anArray = [1 ,2, 3, 4, 5, 6, 7, 8, 9]

How can I transpose this matrix so that the elements of my source array are switched as follows?

let newArray = [1, 4, 7, 2, 5, 8, 3, 6, 9] 

Answer 1

You could take the length for the dimension of the array and map items on a specific index for a new array.

var array = [1 ,2, 3, 4, 5, 6, 7, 8, 9],
    n = Math.sqrt(array.length),
    transposed = array.map((_, i, a) => a[(i % n) * n + Math.floor(i / n)]);
    
console.log(transposed.join(' '));

Answer 2

The approach in the answer you linked to works well in JavaScript too.

For a 3 x 3:

const anArray = [1, 2, 3, 4, 5, 6, 7, 8, 9];

let newArray = [];
for (let i = 0; i < 3; i++) {
  for (let j = 0; j < 3; j++) {
    newArray[3 * i + j] = anArray[3 * j + i];
  }
}

console.log(newArray);

For an N x N, just replace the 3's with N.

This answer avoids division and flooring (integer division) and a decent optimizer should make the code relatively fast. You might also consider initializing the new array with

let newArray = new Array(9);

or

let newArray = new Array(N * N);

but profile the code before attempting "optimizations" such as this.