Возвращает разность массивов

diff.js

let row = [1,2,3,4,5,6,7]
let films = [4,5,6,7,8,8,9]



function diff(row,films) {
  let out_arr = [];
  films.forEach(element => {
    let idx = row.indexOf(element);
    if(idx < 0) out_arr.push(element);
    console.log(`элемент ${element} - idx: ${idx}`);
  });
  return out_arr
}

let dif = diff(row,films);

console.log(dif);