Check if data exist in Firebase database with Angular

Question

I'm working on web project with Angular connected with Firebase console and I used this function defined in my service class to verify if the value exists in the database before saving, When I call this function in my component I usually get undefined value.

This is my service function :

  ifExist(category : CategoryType){
    firebase.database().ref("/categories/").child("categories").orderByChild("category_name").equalTo(category.category_name)
.once( "value" , snapshot => {
  if (snapshot.exists()){
    const userData = snapshot.val();
    console.log("exists!", userData);
    return true;
  }
  return false;
});  
}

Please read [Under what circumstances may I add “urgent” or other similar phrases to my question, in order to obtain faster answers?](//meta.stackoverflow.com/q/326569) - the summary is that this is not an ideal way to address volunteers, and is probably counterproductive to obtaining answers. Please refrain from adding this to your questions. — halfer, Jul 13 '20 at 08:50

score 1 · Accepted Answer · answered Jul 12 '20 at 21:59

Data is loaded from Firebase asynchronously. Your return false runs before your if (snapshot.exists()){ is called, so you'll always return false.

The solution is to return a promise:

ifExist(category: CategoryType) {
  return firebase.database().ref("/categories/").child("categories")
    .orderByChild("category_name").equalTo(category.category_name)
    .once("value", snapshot => {
      if (snapshot.exists()) {
        const userData = snapshot.val();
        console.log("exists!", userData);
        return true;
      }
      return false;
    });
}

And the use that when calling the function, either with:

ifExist(yourCategoryType).then((result) => {
  console.log("ifExist returned "+result);
});

Or with more modern async / await:

const result = await ifExist(yourCategoryType)
console.log("ifExist returned "+result);

Also see:

orderByChild not working in Firebase
asynchronous callback returns null
Wait until asynchronous operation (firebase) call is done before page loads
Asynchronous Firebase query to response
and probably many more from these search results

hello @Frank sorry but it's not working I usually get null as result in my component — karim, Jul 14 '20 at 08:19

Check if data exist in Firebase database with Angular

1 Answers1