flattenArray.js created by matchilling - https://repl.it/IhOJ/0

flattenArray.js.js

const arr = [
        1, 2, 3, 4, 5, 6, [1, 2, 3, 4], [1, 2, 3, 4], [ [1, 2, [['x']], 4], [1, () => 2, 3, 4], [1, 2, {}, 4, [1, 2, 3, 4]]]
      ],
      flatten = arr => arr.reduce((acc, val) => acc.concat(
        Array.isArray(val) ? flatten(val) : val), []
      );

console.log(
  flatten(arr)
);