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Looks like you're referencing an undefined, namely title.title. You're probably looking to just reference title.id and title.en. delete one of the titles like so:

let titles = {}

this.user.titles.forEach(title => {
    let key = title.year_id

    titles[key] = {}
    titles[key].id = title.id // <-- Changed
    titles[key].en = title.en // <-- Changed
})

return titles

EDIT: Then again, maybe I'm mistaken, as title.title.id would throw a TypeError if title.title were undefined, unless title.title exists like so:

{
  year_id: 0,
  title: "some_title",
  id: 0,
  en: "en?"
}

In which case title exists, but isn't an object, so no TypeError is thrown.