如何返回文档数据 Firebase 数据库？

Question

I'm learning React/Firebase and I have a function that queries parent node for a specific user.

export const fetchUserDocument = (userId) => {
  const db = database.ref("users");
  //if I write return here it outputs
  //  const user = snapshot.val();
  //  return user;
  db.child(userId).on("value", (snapshot) => {
    const user = snapshot.val();
    return user;
  });
  //return db 

};

Then in another file I can call on this function to retrieve user data like this const user = await fetchUserDocument(userId) but I am getting undefined when I console.log(user) .

Answer 1

onSnapshot is asnynchronous, so it cannot return value to you immediately. You need to pass a callback function to fetchUserDocument and call it when you got the user data:

export const fetchUserDocument = (userId, cb) => {
  const db = database.ref("users");

  db.child(userId).on("value", (snapshot) => {
    const user = snapshot.val();
    cb(user);
  });
};

fetchUserDocument(userId, (user) => {
    console.log(user);
});

Another approach would be returning a promise from fetchUserDocument:

export const fetchUserDocument = async (userId) => {
    return new Promise((resolve, reject) => {
        const db = database.ref("users");

        db.child(userId).on("value", (snapshot) => {
            const user = snapshot.val();
            resolve(user);
        });
    });
};

This would give you the result you want:

const user = await fetchUserDocument(userId);
console.log(user);