CODEWITHSUNDEEP
javascript
engineer-x
engineer-x

Reputation: 3183

Can't iterate through array from an async Javascript function?

I'm stuck on an issue where I'm parsing the results from an API in getSubscriptions(). This calls getUserSubs() which returns the following object:

expected result

When I call on subscriptions I get the expected array console (See "Works") snippet.

But when I try to iterate on the array subscriptions (See "Doesn't work"), then the contents of the function are not even called.

userSubs() is called for API data

async function getUserSubs(userId) {
  const ref = collection(db, "users", userId, "subscriptions")
  let subList = []
  try {
    const subIds = await getDocs(ref)
    subIds.forEach(subRef => {
      const docRef = subRef.data()
      getDocData(docRef.product).then(product => {
        subList.push(product)
      })
    })
    return subList

  } catch (e) {
    console.error("Error in getting subscriptions: ", e)
    return []
  }
}

Works

function getSubscriptions(userId) {
  getUserSubs(userId).then(subscriptions => {
    console.log(subscriptions)  // Works as intended 
  }
}

Doesn't work

function getSubscriptions(userId) {
  getUserSubs(userId).then(subscriptions => {
    subscriptions.forEach(x => {
      console.log(x)  // ISSUE: This isn't called 
    })
}

Also doesn't work

let a = []
getUserSubs(userId).then(subscriptions => {
  subscriptions.forEach(x => a.push(x))
})
console.log(a)

I know there are similar questions asked but after reading them I'm still not able to resolve my issue.

Similar issues:

Upvotes: 1

Views: 291

Answers (2)

Bergi
Bergi

Reputation: 664936

getUserSubs(userId).then(subscriptions => {
    console.log(subscriptions)  // Works as intended 
}

No it doesn't. It only appears so because you are inspecting the live array that was mutated after it has been logged to the console.

Also doesn't work:

let a = []
getUserSubs(userId).then(subscriptions => {
  subscriptions.forEach(x => a.push(x))
})
console.log(a)

Yes, for rather obvious reasons: the array is logged before you fill it. It would need to be either

getUserSubs(userId).then(subscriptions => {
  let a = []
  subscriptions.forEach(x => a.push(x))
  console.log(a)
})

or

let a = []
const subscriptions = await getUserSubs(userId)
subscriptions.forEach(x => a.push(x))
console.log(a)

But none of these will solve your core problem: getUserSubs returns an empty array before it gets filled, in the lines

subIds.forEach(subRef => {
  const docRef = subRef.data()
  getDocData(docRef.product).then(product => {
    subList.push(product)
  })
})
return subList

you never wait for the getDocData promise. If you change that to

let subList = []
for (const subRef of subIds) {
  const docRef = subRef.data()
  const product = await getDocData(docRef.product)
  subList.push(product)
}
return subList

or just

return Promise.all(subIds.map(subRef => {
  const docRef = subRef.data()
  return getDocData(docRef.product)
}))

it would work, as described in the question you already found.

(This might still not work. subIds looks suspiciously like a firebase snapshot, which is not an array and can neither be iterated nor does it have a .map method. In that case, you'll need to use forEach+push manually).

Upvotes: 1

Edward
Edward

Reputation: 435

Honestly I wouldn't use the forEach() method. I think all you need to do to fix this is iterate over the results in a normal for loop.

for(let subscription of subscriptions) {
   console.log(subscription);
}

OR

for(let index in subscriptions) {
   console.log(subscriptions[index]);
}

If this doesn't do the trick, I'll open up a sandbox and look more in depth.

Upvotes: 0

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