Javascript 异步函数返回 then-catch 承诺？

Question

Introduction

I am new in the world of javascript promises and I need to understand how they work correctly...

Until know I have been doing this in my code:

  const handleRefresh = () => {
    setIsRefreshing(true);

    fetchUserData()
      .then(async () => { <--------- Using then because I return a promise in fetchUserData
         await fetchUserPosts(); <------- Using await here
         setIsRefreshing(false);
      }).catch(err => { <--------- This will catch the error I have thrown in the function fetchUserPosts or inside the then body
       // TODO - Show error
       setIsRefreshing(false);
       console.log(err)
    );
  };

  const fetchUserData = async () => { <------ async function
    const { firebase } = props;

    const userId = firebase.getCurrentUser().uid;

    const documentRef = firebase.getDatabase().collection("users").doc(userId);

    // Fetch all the user information
    return documentRef <--------- Returning the promise here
      .get()
      .then((doc) => {
        if (doc.exists) {
          // Get all user data
          const data = doc.data();
          console.log(data);
          setUserData(data);
        }
      })
      .catch((err) => {
        throw err; <----------- Throwing error
      });
  };

I don't know if I am doing anti patterns... But basically I need to know if this is a good way and if I am doing this correctly.

Questions

1. Do I have to declare the fetchUserData function as async to return a promise?

2. Can I use the async await in the then/catch body?

3. Can I do this?

    const handleRefresh = async () => { setIsRefreshing(true); await fetchUserData() .then(async () => { <--------- Using then because I return a promise in fetchUserData await fetchUserPosts(); <------- Using await here }).catch(err => { <--------- This will catch the error I have thrown in the function fetchUserPosts or inside the then body // TODO - Show error console.log(err) ); setIsRefreshing(false); };

I would really appreciate if someone guides me. Thanks.

Answer 1

the words async and await are only syntactic sugar for then and catch .

This:

  fetchUserData()
    .then(data => return data )
    .catch(error => return error)

is equivalent to:

 async function getUserData() {
   const userData = await fetchUserData()

   return userData
 }

Here you are returning anything (success or error). If you want to treat the error here, just put a try/catch clause.

  async function getUserData() {
   try {
     return await fetchUserData()
   } catch (e) {
     return e.message
   }
 }

Note that you can only use the await clause within an async function.

Answer 2

1.
Function can return Promise without being declared as async as long as you don't await inside it,

2.
You should not use async-await inside then , simply return a Promise and it'll be resolved in the following then ,

3.
When using async-await syntax, Promises are awaited in a declarative way as demonstrated below:

const handleRefresh = async () => {
  try
  {
    const a = await getA()
    // pass the result to another Promise
    const b = await getB(a)
    const c = await getC(b)
  
  } catch (error)
  {
    handleError(error)
  } 
};