SOURCE

//1
let arr = [1, [2, [3, [4, 5]]]];
function flatten(arr){
    let res = [];
    for(let i = 0; i < arr.length; i++){
        if(Array.isArray(arr[i])){
            res = res.concat(flatten(arr[i]))
        }else{
            res.push(arr[i])
        }
    }
    return res;
}
console.log(flatten(arr))

//2
function flatten2(arr){
    return arr.reduce((t, v) => {
        return t.concat(Array.isArray(v) ? flatten2(v) : v);
    }, [])
}
console.log(flatten2(arr));

//3
function flatten3(arr){
    while(arr.some(i => Array.isArray(i))){
        arr = [].concat(...arr);
    }
    return arr
}
console.log(flatten3(arr));

//4
function flatten4(arr){
    let str = arr.toString();
    return str.split(',').map(Number)
}
console.log(flatten4(arr));

//5
function faltten5(arr){
    return arr.flat(Infinity);
}
console.log(faltten5(arr))