explanation.js

const property = obj[key];
// obj[key] here will be '1991'.

acc[property] = acc[property] || [];
// At this point acc['1991'] doesn't yet exist, so it will be an empty array. This step is important as it checks if acc['1991'] exists, and if not, creates it and assigns a value of an empty array.

acc[property].push(obj);
// Here, all we're doing is pushing our object into the right group