SOURCE

let arr = [1,2,3,[3,4,5,[8,9,76,[3,3,[1]]],3,3,4],3]

while(arr.some((item)=>Array.isArray(item))){
    arr = [].concat(...arr)
}
while(arr.some((item)=>Array.isArray(item))){
    arr = [].concat(...arr);
}
console.log(arr)