const arr = [1, [2, [3, 4]], 5];

function flattenArr(arr){
    let fArr = [];

    for(const a of arr){
        if(Array.isArray(a)){
            fArr = fArr.concat(flattenArr(a))
        } else{
            fArr.push(a);
        }
    }
    return fArr;
}

const result = flattenArr(arr);
console.log(result,'---result')