flatten_array.js

function flatten(array) {
  let i = 0;
  while (i < array.length) {
    if (!Array.isArray(array[i])) {
      i++
    } else {
      array.splice(i, 1, ...array[i])
    }
  }
}

const errorMsg = 'array is not flatten';
function test() {
  let array = [[1,2,[3]],4]
  flatten(array)
  const shouldBe = [1,2,3,4]
  console.assert(arraysEqual(array, shouldBe), {array, errorMsg})
}

// https://stackoverflow.com/questions/3115982/how-to-check-if-two-arrays-are-equal-with-javascript/16436975#16436975
function arraysEqual(a, b) {
  if (a === b) return true;
  if (a == null || b == null) return false;
  if (a.length != b.length) return false;

  // If you don't care about the order of the elements inside
  // the array, you should sort both arrays here.
  // Please note that calling sort on an array will modify that array.
  // you might want to clone your array first.

  for (var i = 0; i < a.length; ++i) {
    if (a[i] !== b[i]) return false
  }
  return true
}

test()