double return 是有效的语法吗？

Question

Hi I am reading a blog on a certain website.

In his code he is loading data from app state using firebase. As you can see in his code. He is making double return statement. Is that a valid syntax. if valid what does it return? function or value?

export const loadState = (): AppState => {

  if (firebase.auth().currentUser.uid) {
    let userId = firebase.auth().currentUser.uid;
    return firebase.database().ref('/users/' + userId).once('value').then(function(snapshot) {
        return snapshot.val() as AppState;
      }
    }
  )
}

Answer 1

Let's make your code a little bit better to read

export const loadState = (): AppState => {

  if (firebase.auth().currentUser.uid) {
    let userId = firebase.auth().currentUser.uid;
    return firebase.database()
      .ref('/users/' + userId)
      .once('value')
      .then(function(snapshot) {
        return snapshot.val() as AppState;
      }
    }
  )
}

Now you will see that the second return is inside an inline function which is the successCallback for the Promise which is returned by Firebase.

firebase.database() returns a Promise . The ref() and once() methods are chain functions which receive the Promise and return a new Promise which is then ultimatively passed to the then() chain. Your callback returns a new Promise which then is returned as Appstate to be the return value of the loadState function. So loadState returns a Promise as well.

Read more about Promises on MDN
Read more about Firebase Database API