Use es6 method - Generator, to flatten multidimensional array.

flatten_array.js

// method
const flat = function *(arr) {
  const {length} = arr;
  for(let i = 0; i< length; i++) {
   const item = arr[i];
    if(Array.isArray(item)){
		yield* flat(item);
	} else {
		yield item;
	}
  }
}

// main
const flatten = function (arr) {
	return [...flat(arr)];
}

// try
const array =  [1, [[2, ['a', 'b', ['c', ['d']]], 3], 4], [5, 6]];
console.log(flatten(array)); // [1, 2, "a", "b", "c", "d", 3, 4, 5, 6]