For a better understanding for the second code in the teacher's note

const numbers = [1, 1, 2, 3, 4, 3, 5, 5, 6, 7, 3, 8, 9, 10];
const unique = numbers.filter(function(number, index, array) {
  return index === array.indexOf(number);
});
console.log(unique); // => [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

/** write process one by one
 * (num, index) => return index === array.indexOf(number);
 * (1, 0) => return 0 === array.indexOf(1);  true (new index)
 * (1, 1) => return 1 === array.indexOf(1);  false (exist already)
 * (2, 2) => return 2 === array.indexOf(2);  true (new index)
 * (3, 3) => return 3 === array.indexOf(3);  true (new index)
 * (4, 4) => return 4 === array.indexOf(4);  true (new index)
 * (3, 5) => return 5 === array.indexOf(3);  false (exist already)
 * (5, 6) => return 6 === array.indexOf(5);  true (new index)
 * (5, 7) => return 7 === array.indexOf(5);  false (exist already)
 * (6, 8) => return 8 === array.indexOf(6);  true (new index)
 * (7, 9) => return 9 === array.indexOf(7); true (new index)
 * (3, 10) => return 10 === array.indexOf(3); false (exist already)
 * (8, 11) => return 11 === array.indexOf(8); true (new index)
 * (9, 12) ==> return 12 === array.indexOf(9); true (new index)
 * (10, 13) => return 13 === array.indexOf(10); true (new index)
 */

// returns [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]