flatten.js

const inputArray = [[1, 2, [3]], 4];

const flatten = input => {
  return input.reduce((acc, x) => {
    if (Array.isArray(x)) {
      return [...acc, ...flatten(x)];
    } else {
      return [...acc, x];
    }
  }, []);
};

console.log(flatten(inputArray)); // [1, 2, 3, 4]