flatten an array of arbitrarily nested arrays of integers into a flat array of integers.

flatten.js

/**
* flatten an array of arbitrarily nested arrays of integers into a flat array of integers.
* e.g. [[1,2,[3]],4] -> [1,2,3,4]. 
**/

const isInteger = (n) => {
  if(Number.isInteger(n)) {
    return n;
  };
  throw "An item inside the array is not integer";
}

let flatten = list => {
  if (!Array.isArray(list)) {
    throw "Not an array";
  }
  
  return list.reduce(
      (a, b) => a.concat(Array.isArray(b) ? flatten(b) : isInteger(b)), []
  );
}