May be useful if you need to save empty values

matrixToArray.js

const m = 4;
const n = 3;

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

for (let i = 0; i < n; i++) {
  for (let j = 0; j < m; j++) {
    arr[m * i + j] = matrix[i][j];
  }
}

console.log(arr); // [1, 2, undefined, undefined, 3, 4, 5, undefined, 6, 7, 8, 9]