I don't understand the wording of your question. But from your code example it looks like you're trying to group an array of data by year. Not sure what you're doing with id and en... and I'm assuming year_id is not a unique value.
return this.user.titles.reduce((years, title) => {
const { id, en, year_id } = title;
const yearTitles = years[year_id] || {};
return {
...years,
[year_id]: {
...yearTitles,
[id]: title,
[en]: title,
},
};
}, {});