Not able to return the value while using async function in nodejs

Question

Trying to return the value after google api call in the below async function in google cloud function

 async function getGoogleApiToken() {
      let jwtClient = new google.auth.JWT(
               privatekey.client_email,
               null,
               privatekey.private_key,
               ['https://www.googleapis.com/auth/compute']);
        //authenticate request
       await jwtClient.authorize(function (err, tokens) {
         if (err) {
           console.log(err);
           return;
         } else {
           console.log("Successfully connected!"+ tokens.access_token); //displaying the token correctly
           return tokens.access_token; //returns the token
         }
        });
    }
  exports.consumepromise = async (req,res) =>{
  const tokennew =  await getGoogleApiToken();
  console.log("token"+tokennew);
  }

Error: tokennew is undefined

Not sure where it is going wrong.

You are `await`ing `jwtClient.authorize` but not doing anything with the return value. Your return statement is inside the scope of the callback function you passed to `jwtClient.authorize`, not the scope of your function `getGoogleApiToken`, which returns nothing. — Jake, Apr 26 '20 at 17:22

Mulan · Accepted Answer · 2020-04-27T16:42:31.617

If I had to guess it's because jwtClient.authorize accepts a callback instead of returning a promise -

async function getGoogleApiToken() {
  let jwtClient = new google.auth.JWT(...);
  await jwtClient.authorize(function (err, tokens) {
    if (err) {
      console.log(err);
      return;
    } else {
      console.log("Successfully connected!"+ tokens.access_token); //displaying the token correctly
      return tokens.access_token; //returns the token // <- NO IT DOESN'T
    }
  });
}

Callbacks cannot return a value. This is one of the most popular questions on StackOverflow. Callbacks like the ones used in jwtClient.authorize are just a common usage pattern and do not interop with async and await directly. You must return a Promise from your getGoogleApiToken. One such way is to wrap a Promise around the jwtClient.authorize call -

async function getGoogleApiToken() {
  const jwtClient = new google.auth.JWT(...); // <-- btw use const
  return new Promise((resolve, reject) => // <-- wrap in promise
    jwtClient.authorize((err, tokens) => {
      if (err)
        reject(err) // <-- reject the error
      else
        resolve(tokens) // <-- resolve the result
    })
  )
}

We don't want to add this Promise wrapper every time we wish to use a callback-accepting function with other Promise-based code, including any async functions. Turning any callback-accepting async function into a promise-returning async function can be a mechanical process. In fact, that's what NodeJS's util.promisify does. It works something like this -

// approximation of util.promisify
const promisify = f => (...args) =>
  new Promise ((pass, fail) =>
    f (...args, (err, result) => err ? fail(err) : pass(result))
  )

// example callback-accepting function
const demoCallbackFunc = (a, b, callback) =>
  setTimeout(callback, 1000, null, a + b)

// ordinary call
demoCallbackFunc(3, 5, (err, res) => {
  if (err)
    console.error(err)
  else
    console.log(res)
})

// promisified
promisify(demoCallbackFunc)(3, 5)
  .then(console.log, console.error)

console.log("please wait 1 second...")

Using promisify in your program will clean it up considerably -

async function getGoogleApiToken() {
  const jwtClient = new google.auth.JWT(...);
  const auth = promisify(jwtClient.authorize.bind(jwtClient))
  return auth() // <-- no need for await, auth already returns a promise
}

Or without an intermediate assignment of auth -

async function getGoogleApiToken() {
  const jwtClient = new google.auth.JWT(...);
  return promisify(jwtClient.authorize.bind(jwtClient))()
}

Or without bind using an arrow function -

async function getGoogleApiToken() {
  const jwtClient = new google.auth.JWT(...);
  return promisify(callback => jwtClient.authorize(callback))()
}

@Jax Happy to help. See the link I added in paragraph 2 for more insight. All the best :D — Mulan, Apr 26 '20 at 17:58
@Thankyou In this statement that you have mentioned const auth = promisify(jwtClient.authorize.bind(jwtAuthorize)), what is jwtAuthorize variable here? — Jax, Apr 27 '20 at 15:57
My apologies for the typo, Jax. It is supposed to be `jwtClient.authorize.bind(jwtClient)`. I fixed it in the post too :) — Mulan, Apr 27 '20 at 16:43

score 0 · Answer 2 · answered Apr 26 '20 at 17:30

const tokennew = await getGoogleApiToken(); should be inside an async block or you can use .then and .catch on it like so:

with async block

  async function consumePromise() {
   const tokennew =  await getGoogleApiToken();
   console.log("token"+tokennew);
  }
  // then call it
  consumePromise();

with then and catch

   getGoogleApiToken()
     .then(tokennew => {
       console.log("token"+tokennew);
     })
     .catch(err => {
       console.log(err);
     })

The most important thing to note here is that you can't access a value resolved by a promise outside an async block or without using .then and .catch on the promise.

i have already implemeted didnt added in query, now added the full code — Jax, Apr 26 '20 at 17:35

Not able to return the value while using async function in nodejs

2 Answers2