flatten.js

function flatten(xs) {
    var itr = xs;
    while (true) {
        var ys = [];
        var allFlat = true;
        itr.forEach(x => !(allFlat = (!Array.isArray(x) && allFlat)) ? (ys = ys.concat(x)) : ys.push(x));
        if (allFlat) return ys;
        itr = ys;
    }
}

console.log(flatten([1, 2, [3, 4], [5, [6, [7]]]]));
console.log(flatten([1,[2,[3]],[4,[5,[6,[7]],8],9],10]));