//Method 1
function removeDuplicates(arr) {
  let obj = {};
  let len = arr.length;

  for (let index = 0; index < len; index++) {
    let val = arr[index];
    obj[val] = val;
  }

  return Object.values(obj);
}

console.log(removeDuplicates([21, 15, 0,-1, 2, 21, 15, 9, 7, 6]));
//[0, 2, 6, 7, 9, 15, 21, -1]

//Method 2
function removeDuplicates2(arr) {
  return [...new Set(arr)];
}

console.log(removeDuplicates2([21, 15, 0,-1, 2, 21, 15, 9, 7, 6]));
//[21, 15, 0, -1, 2, 9, 7, 6]