'use strict';
var flattenArr = [];

function flatten(numbers) {
  // Declarative way of solving the problem
  numbers.forEach(function(number) {
    Array.isArray(number) ? flatten(number) : flattenArr.push(number);
  });
  return flattenArr;
}

var unflatternArr = [1, 2, 3, [4, 5], [6, [7, 8]]];
console.log(flatten(unflatternArr)); // [1, 2, 3, 4, 5, 6, 7, 8]