flatten.js

const flatten = function flatten(array) {

  const flattened = array.reduce(

    function reducer(reduced, arrayItem) {

      const arrayItemIsArray = Array.isArray(arrayItem);

      if (arrayItemIsArray) return reduced.concat(flatten(arrayItem));

      return reduced.concat(arrayItem);

    },

    []

  );

  return flattened;

}

const test = function test() {

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

  const expected = '1,2,3,4';

  const flattened = flatten(x);

  if (flattened.toString() === expected) {
    console.log('OK!');
  } else {
    console.error(`FAIL! Expected ${expected}. Got ${flattened.toString()}.`);
  }

  return flatten(x);

}

On Node:

.load flatten.js

Then:

test();

Then:

PROFIT!