Transform 2-D array to 1-D array

flatten-2d-list.hs

flatten2DList :: [[a]] -> [a] -> [a]
flatten2DList (x:[]) acc = acc ++ x
flatten2DList (x:xs) acc = flatten2DList xs (acc ++ x)

-- or just use concat
flatten2DList' :: [[a]] -> [a]
flatten2DList' x = concat x

mapTo1DArray.js

/**
 * @param {Array<Array<unknown>>} arr 2D array to transform into one-dimensional array
 * @return {Array<unknown>|null} one-dimensional array if 2D array was provided, null otherwise
 */
function mapTo1DArray(arr) {
  if (arr == null || !Array.isArray(arr) || !Array.isArray(arr[0])) {
    return null;
  }

  const result = new Array(arr[0].length * arr.length);
  let ptr = 0,
    yPtr = 0,
    xPtr = 0;

  while (ptr < result.length) {
    result[ptr++] = arr[yPtr][xPtr];

    xPtr++;
    if (xPtr > arr[0].length - 1) {
      // last column of current row was reached,
      // reset X pointer to 0 and move to the next row by increasing Y pointer
      xPtr = 0;
      yPtr++;
    }
  }

  return result;
}