flatten.spec.js

const flattenArr = (arr = []) => {
    // Executing recursively a reducer function seems the logical choice, since it will result... 
    // ...in a single output value
    return arr.reduce((accumulator, currentValue) => {
        return accumulator.concat(Array.isArray(currentValue) ? flattenArr(currentValue) : currentValue);
    }, []);
};

// Using Jest for testing
test('[[1,2,[3]],4] -> [1,2,3,4]', () => {
    expect(flattenArr([[1,2,[3]],4])).toEqual([1,2,3,4]);
});

test('[] -> []', () => {
    expect(flattenArr([])).toEqual([]);
});

test('undefined -> []]', () => {
    expect(flattenArr()).toEqual([]);
});