Flatten an array of arbitrarily nested arrays of integers into a flat array of integers

flatArray.js

function flatArray(arrayToBeFlatted, output) {
  if (!arrayToBeFlatted){
    throw new Error('Please provide valid array')
  }
  if (!output) {
    output = [];
  }

  for (let i = 0; i < arrayToBeFlatted.length; i++) {
    const currentArrayItem = arrayToBeFlatted[i]
    if (Array.isArray(currentArrayItem)) {
      flatArray(currentArrayItem, output);
    } else {
      output.push(currentArrayItem);
    }
  }

  return output;
}

console.log(flatArray([[1, 2, [3]], 4]));
console.log(flatArray([[1], [2, 3]]));
console.log(flatArray([[1], [2, 3], [4]]));
console.log(flatArray([[[1, [1]], 2, 3], [4, 5]]));
console.log(flatArray([[1, 2, 3], [4, 5]]))
console.log(flatArray(undefined))
console.log(flatArray(null))