function flattenArray(input) {
    function flatten(input, result) {
        input.forEach((item)=>{
            if (item instanceof Array) {
                result.concat(flatten(item, result))
            } else {
                result.push(item)
            }
        })
        return result
    }
    return flatten(input, [])
}

console.log(flattenArray([[1,2,[3]],4,[[[5,6]],7,8],9]))
console.log(flattenArray([1,2,3,4,5,6]))
console.log(flattenArray([1,2,3,[4,5,[6,7,[8]]]]))
console.log(flattenArray([[1,2,[3]],4]))
console.log(flattenArray([1,2,3,[4,5,[6,7,[8]]], 'foo']))