join with lookup tables on js

users-messages-join.js

const messages = [
  {
    id: 1,
    text: 'text 1',
    userId: 1
  },
    {
    id: 2,
    text: 'text 2',
    userId: 1
  },
    {
    id: 3,
    text: 'text 2',
    userId: 2
  },
    {
    id: 4,
    text: 'text 2',
    userId: 3
  },
];


const users = [
  { id: 1, name: 'user 1' },
  { id: 2, name: 'user 2' },
  { id: 3, name: 'user 3' },
];

const usersLookup = Object.fromEntries(
  users.map( (u) => ( [ [u.id], u ] ) )
)

const messagesByUserId = messages.reduce((prev, curr) => {
  if(curr.userId in prev) {
    prev[curr.userId] = 
      [
        ...prev[curr.userId], 
        {
          ...curr, 
          user: usersLookup[curr.userId]
        }
      ]
  }else {
    prev[curr.userId] = [
      {
        ...curr, 
        user: usersLookup[curr.userId]
      }
    ]
  }
  return prev;
}, {});


console.dir(messagesByUserId,  { depth: null })