flatten.js

const flatten = array => (
  array.reduce((accumulated, current) => accumulated.concat(
    Array.isArray(current) ? flatten(current) : current
  ), [])
);

// Test
const before = [[1,2,[3]],4];
const after = flatten(before);
const expected = [1,2,3,4];
console.assert(after.length == expected.length, `Expected length ${after.length} to be ${expected.length}`)
console.assert(after.every((item, i) => expected[i] == item), `Expected array [${after}] to equal [${expected}]`)