Write asynchronous code in JavaScript Firebase

Question

I'm trying to write some asynchronous code in JavaScript, especially I want the code to be executed after a certain functions has finished the execution.

Below is my code:

 function triggerFunction(duration) {
    var db = firebase.firestore();

    db.collection('users').get().then((querySnapshot) => {
        querySnapshot.forEach((doc) => {
          var data=doc.data();
          db.collection('users').doc(doc.id).collection('#aName').get().then((querySnapshot1) => {
                querySnapshot1.forEach((doc1) => {
                  var firstData = doc1.data()
                  var newID = createRandomId()
                  var alreadyExist = checkAlreadyElementDB(firstData.appType, doc.id)
                  if(firstData.open == true && !alreadyExist){
                      console.log("Entered here: " + alreadyExist)
                      db.collection('#name').doc(newID).set({
                        #NotImportant
                      })
                      db.collection('users').doc(doc.id).collection('#aName').doc(doc1.id).update({
                        open: false
                      })
                  } else if(firstData.open == true && alreadyExist){
                        db.collection('users').doc(doc.id).collection('#aName').doc(doc1.id).update({
                          open: false
                        })
                  }

                })
          })
        });
    });
    setTimeout(triggerFunction, duration);
  }

What I want is first to receive the result from the function

var alreadyExist = checkAlreadyElementDB(firstData.appType, doc.id)

Then to continue to check the following part of the code

      if(firstData.open == true && !alreadyExist){
                      console.log("Entered here: " + alreadyExist)
                      db.collection('#name').doc(newID).set({
                        #NotImportant
                      })
                      db.collection('users').doc(doc.id).collection('#aName').doc(doc1.id).update({
                        open: false
                      })
      } else if(firstData.open == true && alreadyExist){
                        db.collection('users').doc(doc.id).collection('#aName').doc(doc1.id).update({
                          open: false
                        })
                  }

I will really appreciate your help with this problem!

checkAlreadyElementDB() function looks like this:

  function checkAlreadyElementDB(appType, userID){
      var db = firebase.firestore();
      db.collection('#name').get().then((querySnap) => {
        querySnap.forEach((doc) => {
          var opFCData = doc.data();
          if(opFCData.appType == appType && opFCData.userID == userID){
          //  console.log("I HAVE ENTERED HERE")
            return true
          }
        });
      });
      return false
  }

since db.collection('#name').get() is asynchronous, you need to return a Promise from checkAlreadyElementDB, and where you use checkAlreadyElementDB you need to use .then (or aysnc/await) to wait for that promise to resolve — Jaromanda X
– Jaromanda X, Commented Mar 25, 2020 at 0:23
you're already using them in your code db.collection('#name').get() returns a Promise ... as you can see by the .then( .... ) ... anywhere you've used .then in your code, you're using promises — Jaromanda X
– Jaromanda X, Commented Mar 25, 2020 at 0:29
I see, still not sure how to make it work to allow first to execute that function, then to continue with if else... if you have time, I would appreciate a code example so I can see — For Bugged
– For Bugged, Commented Mar 25, 2020 at 1:24

danh · Accepted Answer · 2020-03-25 14:57:28Z

1

The key is to break this logic up into smaller, testable, promise-returning functions. Let's start with checkAlreadyElementDB

// all the functions need this, move it to the top of the file
const db = firebase.firestore();

function checkAlreadyElementDB(appType, userID) {
  return db.collection('#name')
    .where('appType', '==', appType)
    .where('userID', '==', userID).get().then(snapshot => {
      return !snapshot.empty
  })
}

Notice how the check can be made using where clauses on the query. This is a much quicker operation, transferring much less data to the client. Notice also how this and all of the other functions return promises

Next, the center of the OP code took a little effort to untangle. There were nested promises and names that made it tough to follow. I did my best here to understand what the code meant, despite not really understanding the app.

function closeThenSetOrUpdate(doc) {
  let data = doc.data()
  // update the passed doc, no matter what the checkAlready... result is
  return doc.ref.update({ open: false }).then(() => {
    return checkAlreadyElementDB(data.appType, doc.id)
  }).then(exists => {
    if (data.open && !exists) {
      console.log("Entered here: " + exists)
      const newID = createRandomId()
      return db.collection('#name').doc(newID).set({ #NotImportant })
    } else {
      return Promise.resolve() // just an empty promise
    }
  })
}

Here's a function that handles each user by invoking the logic above. Notice that it gathers promises and executes them all together with Promise.all()

function updateANamesForUserWithID(userId) {
  return db.collection('users').doc(userId).collection('#aName').get().then(snapshot => {
    let promises = snapshot.docs.map(doc => {
        return closeThenSetOrUpdate(doc)
    })
    return Promise.all(promises)
  })
}

Now, the core function is simpler to write, using the same Promise.all() technique...

function triggerFunction(duration) {
  return db.collection('users').get().then(snapshot => {
    let promises = snapshot.docs.map(user => {
      return updateANamesForUserWithID(user.id)
    })
    return Promise.all(promises)
  }).then(() => {
    setTimeout(triggerFunction, duration);
  })
}

edited Mar 25, 2020 at 14:57

answered Mar 25, 2020 at 2:00

danh

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3 Comments

For Bugged Over a year ago

One thing which I observed is that here return db.collection('users').doc(doc.id).collection('#aName').get().then(snapshot => { I don't see the declaration of doc.id. Is it supposed to be declared before in the triggerFunction or should I have another one and pass it as a param into triggerFunction?

danh Over a year ago

@ForBugged - see edit. Same technique applied to each user. Caveat is that I can't guarantee that this code even compiles, since it's a big job (as you know to build a rig to test it). Also note, that I'm flying blind on the meanings of the entities and logic (making it harder to keep track). There's a whole other topic to discuss here about naming things in a way that helps future readers (including yourself) makes sense of the code.

For Bugged Over a year ago

thank you for your answer! Now I see how I can go ahead with this. I solved my problem.

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