flatarray.js

// SOLUTION 1 (good performance)
function flat1(source) {
    var s = "[" + JSON.stringify(source).replace(/\[|]/g,'') +"]";
    var flattened = JSON.parse(s);
}

console.log(flat1([[1,2,[3]],4, [[1,2,[3]],4], [[1,2,[3]],4], [[1,2,[3]],4]]);

// SOLUTION 2 (for small arrays)
function flat2(source, pos, prevs, final) {

    let el = source[pos];

    if(Array.isArray(el)) {
        prevs.unshift({
            source, 
            pos
        });
        return flat(el, 0, prevs, final);

    } else if (!isNaN(el)) {
        final.push(el);
        return flat(source, ++pos, prevs, final);

    } else if(prevs.length >= 1){
        const prev = prevs.shift();
        return flat(prev.source, ++prev.pos, prevs, final);

    } else {
        return final;
    }
};

const arr = [[1,2,[3, [5, 7, 6, [3, 2, 4, [4, [3, [2, [1]]]]]]]],4, 2, 1, 5, 6, 3, 3, 1, 5];
const final = flat2(arr, 0, [], []);
console.log(final);