interview question: implement array flatten

flatten.js

// unwraps one level by accumulating the
// array concatenation of one item with the next item
const unwrap = arr => arr.reduce((acc, item) => acc.concat(item), []);

// flattens an array with nested array items
const flatten = arr => {
    const needsUnwrapping = arr.filter( i => Array.isArray(i)).length > 0;
    if (!needsUnwrapping) {
        return arr;
    }
    return flatten(unwrap(arr));
};

// ------

//tests
const a = [[1], [2], [3], [4]];
const b = [1, [2], [[3, [4]]]];
const c = [[1, 2, [3]], 4];
const answer = [1, 2, 3, 4];

const isCorrect = arr => JSON.stringify(arr) === JSON.stringify(answer);

console.log(isCorrect(flatten(a)));
console.log(isCorrect(flatten(b)));
console.log(isCorrect(flatten(c)));