Ways to flatten nested array

Flat.js

//stackoverflow
function flatten(arr) {
  let i = 0;

  if (!Array.isArray(arr)) {
    /* return non-array inputs immediately to avoid errors */
    return arr;
  }

  while (i < arr.length) { 
    if (Array.isArray(arr[i])) {
      arr.splice(i, 1, ...arr[i]);
    } else {
      i++;
    }
  }
  return arr;
}

Flatten.js

var flatten = arr => {
  let res = [];
  
    for(let i = 0;i<arr.length;i++){
      if(Array.isArray(arr[i])) {
        res.push(...arr[i]);
      } else {
        res.push(arr[i]);
      }
      if(i === arr.length-1 && hasArray(res)){
        //reset old arr to new 
        arr = res;
        //clear res
        res = [];
        //restart loop wohoo!
        i=-1;
        continue;
      }
    }

  return res;
}

const hasArray = (arr) => {
  for(let i=0;i<arr.length;i++){
    if(Array.isArray(arr[i])){
      return true;
    }
  }
  return false
}

console.log(flatten([1,2,[3,4,[5,6,7,[8,9,[10,[11,12,[13,14,[15,16]]]]]]]]));

[
   1,  2,  3,  4,  5,  6,
   7,  8,  9, 10, 11, 12,
  13, 14, 15, 16
]

The method using splice is faster...