flatten.js

function flatten(inputArray) {
  /**
   * One can argue that it's a good idea to validate inputArray
   * value here, but i disagree. In this exact case it's better
   * to not reinvent the wheel — if inputArray is not an Array
   * application will throw error anyway at .reduce step
  */

  return inputArray.reduce((accumulator, current) => {
    if (Array.isArray(current)) {
      return [
        ...accumulator,
        ...flatten(current)
      ];
    } else {

      // can also be done via accumulator.concat(current)
      return [
        ...accumulator,
        current
      ];
    }
  }, []);
}

console.log(flatten([1,3,[5, [9,10, [11,22]]]]));
console.log(flatten([]));
console.log(flatten([1,3]));
console.log(flatten([1,3,[5, [9,10]]]));
console.log(flatten([89, [3, 5], 1, [[17, 11]]]));