let arr = [5,9,2,3,7,10,15,20, 11];
// reverse an array in new array
const reverse = function (arr) {
  let revarr = [];
  for(let i= arr.length-1; i>= 0; i--) {
    revarr.push(arr[i]);
  }
  return revarr;
}
// reverse in same arr
const reverse = function(arr) {
  let mid = Math.floor((arr.length+1)/2);
  for(let i = arr.length - 1; i >= 0; i--) {
    if(i>= mid) {
       let temp = arr[(arr.length-1) - i];
       arr[(arr.length-1) - i] = arr[i];
       arr[i] = temp
    }
  }
  return arr;
}

console.log(reverse(arr));