Nodejs 似乎在等待包含 Promise 的函数解析之前返回响应

Question

I am trying to hit my API endpoint 'categories' to return a list of documents from my categories collection in firebase. Using the following code:

function getCategories() {
  let arr = [];
  categories
    .get()
    .then(snapshot => {
      snapshot.forEach(doc => {
        console.log(doc.data().category);
        arr.push(doc.data().category);
        return arr;
      });
    })
    .catch(err => {
      console.log(err);
      return [1, 2, 3];
    });
}

router.route("/categories").get(async function(req, res, next) {
  let arr = await getCategories();
  res.json(arr);
});

While I don't get any errors I receive an empty response which when debugged shows that arr is undefined. The thing is when using the debugger the console logs inside the forEach get called and I see my categories in the console but for some reason, it doesn't wait for the Firebase Promise to resolve and the router just responds with an undefined arr variable.

Answer 1

Your .then callback should look more like this (there is no need for a separate arr variable):

function getCategories() {
  // notice we are returning the promise
  return categories
    .get()
    .then(snapshot => {
      // notice we are returning the new array using .map()
      return snapshot.map(doc => {
        console.log(doc.data().category);
        return doc.data().category;
      });
    })
    .catch(err => {
      console.log(err);
      return [1, 2, 3];
    });
}

And since you are using async/await already, you can rewrite this all to look like this so that your users can know when an error happened:

// notice the function is now async
async function getCategories() {
    const snapshot = await categories.get()
    // this can be a one-liner in the example above too
    return snapshot.map(doc => doc.data().category);
}

router.route("/categories").get(async function(req, res, next) {
   try {
     let arr = await getCategories();
     res.json(arr);
   } catch(ex) {
     res.status(500).json({ msg: ex.message });
   }
});

Answer 2

You are not returning your promise and value inside the .then . Add return in front of categories.get() call and return value inside then.

Answer 3

getCategories needs to return a promise if you want to use await on it. Right now, it's returning nothing. You'll need to return the promise that you created:

function getCategories() {
  let arr = [];
  return categories
    .get()
    .then(snapshot => {
      snapshot.forEach(doc => {
        console.log(doc.data().category);
        arr.push(doc.data().category);
        return arr;
      });
    })
    .catch(err => {
      console.log(err);
      return [1, 2, 3];
    });
}

Note the return keyword.

You also possibly have another problem here. You're iterating the snapshot using forEach , and returning arr from within the lambda function that you passed to forEach . This will not return the snapshot data to the caller. You will need to return a value from the then lambda:

    .then(snapshot => {
      const arr = []
      snapshot.forEach(doc => {
        console.log(doc.data().category);
        arr.push(doc.data().category);
      });
      return arr;
    })

Answer 4

You can try something like this :

function getCategories() {
  let arr = [];
  categories
    .get()
    .then(snapshot => {
      snapshot.forEach(doc => {
        console.log(doc.data().category);
        arr.push(doc.data().category);
        });
        console.log('arr',arr); //check the array data
        return arr;
    })
    .catch(err => {
      console.log(err);
      return [1, 2, 3];
    });
}

router.route("/categories").get(async function(req, res, next) {
  let arr = await getCategories().catch(err => {
         res.status(400).send(err);
   });
  res.json(arr);
});