Cannot figure out how to wait for Promise

Question

I have an array with user IDs and I need to find out what name belongs to each ID and return them in an array. I can get the user's name from the database, using knex and push them into an array, but when I try to send the data it is always an empty array.

I am not really good with Promises so can't figure out how to apply to my project.

const userId = [10,11,12,13]
let users = []

userId.map(id => {
    db.select('name').from('users').where('user_id', id)
    .then(user => {
        users.push(user)
    })
})
res.json(users)

I want the response to wait until the looping finishes and send the users array.

i can help u with the ans but trust me start reading about await/async, Promises try this https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise a good dev always reads — just coding, Aug 19 '19 at 17:18

score 3 · Accepted Answer · answered Aug 19 '19 at 17:08

Your map is creating an array of undefined because your callback function doesn't return anything. If we tweak it slightly, it'll create an array of promises, which conveniently is exactly what Promise.all expects. :-) So:

const userId = [10,11,12,13]
Promise.all(
    userId.map(id => db.select('name').from('users').where('user_id', id))
)
.then(users => {    // `users` is an array of users, in the same order as the IDs
    res.json(users);
})
.catch(error => {
    // Render an error response
});

[worth mentioning](https://stackoverflow.com/questions/28889450/when-should-i-use-return-in-es6-arrow-functions) — Jonas Wilms, Aug 19 '19 at 17:56

score 1 · Answer 2 · edited Aug 19 '19 at 17:20

First you need to wait for all promises to finish before running res.json(...)

Second, you shouldn't mutate outside variables after promise resolving (the order by which the promises resolve will alter your output and that is not nice.

Something like this should work fine

const userId = [10,11,12,13]

// map userId array to promise array
// Promise.all aggregates a promise array into one big promise that resolves when all promises resolve (and preserves array order)
Promise.all(
  userId.map(id =>
    db
      .select("name")
      .from("users")
      .where("user_id", id)
  )
)
  .then(users => res.json(users))
  .catch(e => console.error("Error::", e));

/*handle error in the catch block*/

/* visual example of Promise.all.then block
Promise.all([           users = [
   getUser(10),    ->     {userId: 10, ....}
   getUser(11),    ->     {userId: 11, ....}
   getUser(12)     ->     {userId: 12, ....}
])                      ]

*/

@JonasWilms depending on the environment some require to `return` the Promise so it doesn't hang the request (e.g. firebase functions) but you are right, habits. — Carlos Sá, Aug 20 '19 at 08:49

score 1 · Answer 3 · answered Aug 19 '19 at 17:15

As an alternative answer, here's how you could make 1 trip to the DB for this particular query meaning you don't need to wait for multiple Promises and reduce the load on your DB

knex.raw(
  'select name from users where user_id in (' + userId.map(_ => '?').join(',') + ')', 
  [...userId]
);

score 0 · Answer 4 · answered Aug 19 '19 at 17:28

You want Promise.all(), see here.

Try

const userId = [10,11,12,13]

let users = userId.map(id => new Promise(resolve => {
    db.select('name').from('users').where('user_id', id)
    .then(user => {
        resolve(user)
    })
}))
Promise.all(users).then(()=>res.json(users))

Here users is an array of promises. As soon as all of them are resolved, do res.json(users).

Cannot figure out how to wait for Promise

4 Answers4