const obj = [{id: 'a', time: 1}, {id: 'a', time: 2}, {id: 'b', time: 1}];
 
const objMapped = obj.reduce((acc, item) => {
  let { id, time } = item;
  acc[id] = acc[id] || [];
  acc[id].push(time);
  return acc;
}, {});
 
console.log(objMapped);