flatten.js

function flatten(intArr, flatArr) {
    var results = flatArr || [];

    if (intArr && intArr.length > 0) {
        intArr.forEach(function(value) {
            if (typeof value === 'number') {
                results.push(value);
            } else if (value instanceof Array) {
                flatten(value, results);
            }
        });
    }

    return results;
};


// Test cases
console.log(flatten([[1,2,[3]],4]))
console.log(flatten([[[1,2],[3]],4]))
console.log(flatten([[1,[2,[3]]],4]))
console.log(flatten([[1,2,[3]],[4,8,9]]))
console.log(flatten([[1,2,[3]],[4, [4,6]]]))