承诺和异步/等待组合，等待异步操作的理想方法？

Question

Having async/await by itself in a function would return a "pending value" but if a promise used then the actual value will be returned in the end. Is this the ideal way to wait for the completion of asynchronous operations?

    var foundForm = await getDocument(query) //Returns the resulting document


 async function getDocument(query){
 return new Promise((resolve,reject) =>{
 MongoClient.connect  (url, async function(err, db) {
if (err) throw err;
 console.log(query)
db.collection("users").find(query).toArray(function(err, result) {
  if (err) {
    console.log(err)
    throw err;
  }
  console.log(result);
  db.close();
  resolve(result) //   returns result;


       });
     });
  })
 }

      var foundForm = await getDocument(query) //Returns 'pending'


    async function getDocument(query){

      MongoClient.connect  (url, async function(err, db) {
        if (err) throw err;
    console.log(query)
        db.collection("users").find(query).toArray(function(err,                            result) {
    if (err) {
    console.log(err)
    throw err;
  }
  console.log(result);
    db.close();

   return result;
     });

     })

}

Answer 1

Since your getDocument code needs to wait for an asynchronous operation that doesn't provide a Promise interface, getDocument shouldn't be an async function, because you need to create the promise manually. (And the callback you give to a non-promise-focussed function should almost never be an async function.)

function getDocument(query){
    return new Promise((resolve,reject) =>{
        MongoClient.connect(url, function(err, db) {
            if (err) {
                // Reject, don't throw
                reject(err);
                return;
            }
            console.log(query);
            db.collection("users").find(query).toArray(function(err, result) {
                if (err) {
                    // Reject, don't throw
                    reject(err);
                    return;
                }
                console.log(result);
                db.close();
                resolve(result);
            });
        });
    });
}

Alternately, use promise-enabled versions of MongoClient.connect and db.collection("users").find . MongoDB has those available in its JavaScript API now (I'm afraid I don't have the details). Then you'd use an async function with await , something like this (according to this blog post ):

// BE SURE TO DOUBLE-CHECK THE DETAILS
async function getDocument(query){
    const db = await MongoClient.connect(url);
    const await result = db.collection("users").find(query).toArray();
    console.log(result);
    await db.close(); // No idea whether you need `await` here or not
    return result;
}

Answer 2

In short, yes. If you are targeting a platform that supports es6 async/await and want to utilize that, then if a library only exposes a callback api then "wrapping" the callback in a Promise (your first function) is how you'd accomplish that. Although you'd first want to make sure that the library doesn't offer Promise based api, then you could just return the Promise from the library and await that.