refactoredCompare.js

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

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

let foo = [].compare(numbers, otherNumbers);
console.log(foo);