How to wait for an async query to finish in a for loop?

Question

I want to get the result of an async function - depending on the result I either want to execute another function or continue the for loop. Here is my Cloud Function:

return query.get().then(geoSnapshot => {
        // If GeoquerySnapshot returns 0 docs, createDoc()
        let docs = geoSnapshot.docs;
        if (docs.length === 0) return createDoc(data);
        for(let [index, doc] of docs.entries()){
            // If age preferences are valid, joinDoc
            let currentDoc = docs[index].data();
            if (validAge(currentDoc, data)){
                let matchedBefore = await(matchedBefore(currentDoc, data))
                if (!matchedBefore){
                    return joinDoc(docs[index], data);
                } else if (index === (docs.length - 1)) {
                    return createDoc(data);
                }
            }
            return
        }
    }).catch( error => {
        console.log("error query: " + error);
        return { error: error };
    })

async function matchedBefore(currentDoc, data){
    return db.collection('users').doc(data.uid).get().then( doc => {
        if ( !doc.exists ) return false;
        // If user1 in matchedUsers
        let matchedUsers = doc.get('matchedUsers');
        if (matchedUsers === undefined) return true
        let matchedBefore = matchedUsers.includes(currentDoc.user1);
        console.log('matchedBefore: ' + matchedBefore);
        if (matchedBefore) { 
            return false 
        } else {
            return true
        }
    })
}

I'm getting this following error on let matchedBefore = await(matchedBefore(currentDoc, data)) :

Each then() should return a value or throw

How can I ensure the function matchedBefore() finishes before the for loop continues?

Answer 1

I think you are too confused with the implementation of async-await. The correct way would be:

 return query.get().then(async geoSnapshot => { // If GeoquerySnapshot returns 0 docs, createDoc() let docs = geoSnapshot.docs; if (docs.length === 0) return createDoc(data); /* eslint-disable no-await-in-loop */ for(let [index, doc] of docs.entries()) { // If age preferences are valid, joinDoc let currentDoc = docs[index].data(); if (validAge(currentDoc, data)){ let matchedBefore = await(matchedBefore(currentDoc, data)) if (!matchedBefore){ return joinDoc(docs[index], data); } else if (index === (docs.length - 1)) { return createDoc(data); } } return } /* eslint-enable no-await-in-loop */ }).catch( error => { console.log("error query: " + error); return { error: error }; }) async function matchedBefore(currentDoc, data){ let doc = await db.collection('users').doc(data.uid).get() if ( !doc.exists ) return false; // If user1 in matchedUsers let matchedUsers = doc.get('matchedUsers'); if (matchedUsers === undefined) return true let matchedBefore = matchedUsers.includes(currentDoc.user1); console.log('matchedBefore: ' + matchedBefore); return !matchedBefore }