How to await a promise?

Question

I just started learning NodeJS, and pushed by c# knowledge I'm trying the async / await operator, anyway, I need to ask how can I await a promise to get the result, in particular:

const salt = await bcrypt.genSalt(10);
const hash = await bcrypt.hash(user.password, salt);

the code above generate an hash using the user password and a salt. This works like a charm, but suppose that I want break the code as the following:

 const hash = await bcrypt.genSalt(10, async (err, salt) =>  {
        return await bcrypt.hash(user.password, salt);
 });

I will get undefined, what I did wrong?

Is the first or second version better? Just to start me on this way.

Thanks in advance.

Answer 1

It is necessary a clarification:

• an async return a Promise
• await resolve "thenable" functions, like Promise

So, using the await in a callback-style can't works.

This is an example with your mixed-up callback + async/await in order to understand better how they work.

function getMyHash(password) {
  return new Promise((resolve, reject) => {
    bcrypt.genSalt(10, async (err, salt) => {
      if (err) {
        reject(err)
        return
      }
      try {
        const hash = await bcrypt.hash(password, salt);
        resolve(hash)
      } catch (err) {
        reject(err)
      }
    });
  })
}

function async doSomething() {
  const hash = await getMyHash(user.password)
}

I suggest you to choose one pattern and not mixing up together because could become really hard to read and affect performance (callback are always faster then promise)

Answer 2

You are mixing up callback and async/await which are two different tools used to deal with asynchronous calls.

---

* async/await pattern *

Here we are calling genSalt() which returns a Promise object that you resolve (wait for the operation to be done).

const hash = await bcrypt.genSalt(10);

---

* callback pattern *

Here we are calling genSalt() which will call the provided function as callback when finished.

bcrypt.genSalt(10, () => {
   // Callback
});

---

Theses are two distinct method to handle asynchronous functions.

---

---

So the following example are working :

// async/await pattern
const salt = await bcrypt.genSalt(10);
const hash = await bcrypt.hash(user.password, salt);

---

// callback pattern
function generate(callback) {
   bcrypt.genSalt(10, (salt) => {
     bcrypt.hash(user.password, salt, (hash) => {
        callback(hash);
     });
   });
}

---

---

EDIT : To answer "Is the first or second version better?"

---

Javascript have history. Every year a new version of the norm ECMA is made up, like ECMA2015, ECMA2016, ECMA2017 ...

In vanilla js they were callbacks, in ES5 they were promises, in ES6 they were async/await.

async/await are the future. Be a part of the future!