[1,2,[3,[4,5]]].flat(Infinity) //[1, 2, 3, 4, 5]

var arr = [[1, 2, 23], [13, 4, 5, 5], [6, 7, 9, [11, 12, [12, 13, [14]]]], 10];
 var result = [];
 function flatFn(arr,res=[]) {
   arr.forEach(item => {
    if (Array.isArray(item)) {
     flatFn(item,res);
    } else {
     res.push(item)
    }
   })
  return res;
 }
 result = flatFn(arr);
 console.log("result", result)