flatten.js

// flattens an array of arbitrarily nested arrays of integers into a flat array of integers
const flatten = (values) => values.reduce(
    // concat single values and recursively flatten any nested arrays
    (acc, val) => acc.concat(Array.isArray(val) ? flatten(val) : val), []
);

export default flatten;

flatten.test.js

import flatten from './flatten';

it('flattens an array of arbitrarily nested arrays of integers into a flat array of integers', async () => {
    const arr = [[1,2,[3]],4];
    const flatArr = flatten(arr);
    const innerArrs = flatArr.filter((item) => Array.isArray(item));
    expect(innerArrs.length).toBeLessThan(1);
});