comparison.js

let numbers = [1,2,3,4,5,6,7,8,9];
let otherNumbers = [18, 36, 4, 2, 19, 8, 7, 16, 45];
let uniqueNumbers = [];

  compareArrays = (firstArray, secondArray) => {
    return firstArray.forEach((number) => {
      let comparison = secondArray.some((otherNumber) => {
        return number === otherNumber;
      });
       !comparison ? uniqueNumbers.push(number) : null;
    });
  };

compareArrays(numbers, otherNumbers);
console.log(uniqueNumbers);