Firebase 函数在同一个 onCall 中读取和添加数据 function

Question

I have a function that is triggered when I click a button. Based on the docId I can retrieve the right download URL in Firestore. But I also want to update a specific field (availableDownloads) in my "customers" collection. I can't get it to work.

This code works fine. It returns the download url.

exports.getDownloadUrl = functions.https.onCall(async(data, context) => {
    var docRef= await db.collection('projects').doc(data.docId);
    return docRef.get().then(function(doc){
        const downloadURL = doc.data().downloadURL;
        return downloadURL;
    }).catch(function(error) {
        // Handle error
    });
});

However this doesn't. It returns null

exports.getDownloadUrl = functions.https.onCall(async(data, context) => {
    var docRef= await db.collection('projects').doc(data.docId);
    return docRef.get()
    .then(async function(doc){
        const downloadURL = doc.data().downloadURL;
        const userRef = await db.collection('customers').doc(context.auth.uid);
        return userRef.update({
            availableDownloads: admin.firestore.FieldValue.increment(-1)
        }).then(()=> {
            return downloadURL;
        }).catch((error)=> {
            
        })
        
    }).catch(function(error) {
        // Handle error
    });
});

Answer 1

The first thing that jumps out at me is that .update() is an asynchronous function so you need an await in front of userRef.update(...) .

You should also consider sticking to either arrow functions ( () => {} ) or anonymous functions ( function () {} ), but that's just a stylistic note. :)

ETA: as Frank van Puffelen pointed out, you don't need await s for simply declaring a DocumentReference since those are not async functions. (looking at the line var docRef= await db.collection('projects').doc(data.docId); ).

Answer 2

In addition to the potential problems due to the mix of async/await with then/catch , your main problem is due to variable scoping . The downloadURL variable is a local variable declared inside the first then() block and is therefore not accessible in the second then() block.

You can easily see the problem with the following code:

  function later(delay, value) {
    return new Promise((resolve) => setTimeout(resolve, delay, value));
  }

  later(500, 'value1')
    .then((value) => {
      console.log('First then block: ' + value);
      const downloadURL = value;
      return later(1000, 'value2');
    })
    .then((value) => {
      console.log('Second then block: ' + value);
      console.log('Second then block: ' + downloadURL); // Here it is going to throw an error
    })
    .catch((error) => {
      console.log(error);
    });

---

You need to declare the downloadURL variable in the scope of the Cloud Function, as follows:

exports.getDownloadUrl = functions.https.onCall((data, context) => {
    let downloadURL;
    const docRef = db.collection('projects').doc(data.docId);
    return docRef.get()
        .then((doc) => {
            downloadURL = doc.data().downloadURL;
            const userRef = db.collection('customers').doc(context.auth.uid);
            return userRef.update({
                availableDownloads: admin.firestore.FieldValue.increment(-1)
            });
        })
        .then(() => {
            return downloadURL;
        }).catch((error) => {
            // Handle error
        })
});

---

Note that the above code does not use async/await. If you want to use async/await, do as follows:

exports.getDownloadUrl = functions.https.onCall(async (data, context) => {

    try {
        const docRef = db.collection('projects').doc(data.docId);

        const doc = await docRef.get();
        const downloadURL = doc.data().downloadURL;

        const userRef = db.collection('customers').doc(context.auth.uid);
        await userRef.update({
            availableDownloads: admin.firestore.FieldValue.increment(-1)
        })

        return downloadURL;
    } catch (error) {
        // Handle error
    }

});

You can see that with this code, due to the fact that it is simplified by using async/await, there is no more complexity due to variable scoping.