pure-flatten.js

// flatten.js

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

module.exports = flatten;

// flatten-test.js

const flatten = require('./flatten');
describe('flatten', () => {
  it('[[1,2,[3]],4] should return [1, 2, 3, 4]', () => {
    expect(flatten([[1,2,[3]],4])).toEqual(expect.arrayContaining([1, 2, 3, 4]));
  });

  it('[[[[1]]]] should return [1]', () => {
    expect(flatten([[[[1]]]])).toEqual(expect.arrayContaining(([1])));
  });
});

// the code run in https://repl.it/repls/HelplessLoudScripts