test2.js

const input1 = [1, 1, 2, 2, 3, 3, 8, 4, 4];

const onlyOnce = (input) => {
  const result = [];

  input.filter((item) => {
    let times = 0;
    input.forEach((item2) => {
      if (item === item2) {
        times += 1;
      }
    });
    if (times === 1) {
      result.push(item);
    }
  });
  return result[0];
};

console.log(onlyOnce(input1));

test3.js

const input1 = "Hello world this is a test";
const k1 = 3;

const truncate = (input, k) => {
  const words = input.split(" ");
  return words.filter((word, index) => index <= k - 1).join(" ");
};

console.log(truncate(input1, k1));