How can I await data from a callback function and then return it?

Question

I am in a asynchronous function, trying to return the data from a callback function:

async function getData(){
    const client = new google.auth.JWT(
        keys.client_email,
        null,
        keys.private_key,
        ['https://www.googleapis.com/auth/spreadsheets']    
    )

    let returnData
    const finalData = (data) => {
        returnData = data
    }

    async function gsrun(client) {
        const gsapi = google.sheets({version:'v4', auth: client})

        const options = {
            spreadsheetId: '1d3ZiP1I9jJ2ddlD1Hx2ylWn1VFD_5lYQ9Ps9e9gEqI',
            range: 'Sheet1!A1:H5'
        }

        const data = await gsapi.spreadsheets.values.get(options)

        return data.data.values
    }

    client.authorize( async (err, tokens)=> {
        if(err) return console.log(err)
        let data = await gsrun(client)
        finalData(data)
    })

    return returnData
}

And in the console I get: Promise { undefined }. How should I await that promise to resolve or to await the data?

Does this answer your question? [How do I return the response from an asynchronous call?](https://stackoverflow.com/questions/14220321/how-do-i-return-the-response-from-an-asynchronous-call) — CertainPerformance, Mar 10 '20 at 08:18
and https://stackoverflow.com/questions/22519784/how-do-i-convert-an-existing-callback-api-to-promises and https://stackoverflow.com/questions/23667086/why-is-my-variable-unaltered-after-i-modify-it-inside-of-a-function-asynchron — CertainPerformance, Mar 10 '20 at 08:18
I followed the documentation but is not what I need. It says nothing about returning the data from a async callback function. — , Mar 10 '20 at 08:47

score 0 · Answer 1 · answered Mar 10 '20 at 08:57

Your client.authorize is already awaiting gsrun() function to return a resolve (I am not sure how the rejection from gsrun() is handled but I suggest you use a .then() to handle resolve/reject if not implement a try,catch statement).

Another issue is that following code will still run despite you awaiting gsrun(client)

return returnData

because it is not inside the finalData() function. I suggest you only execute it is set.

Here's to show that returnData is returned before data is assigned to it

function returnPromise() {
    return new Promise((resolve, reject) => {
      setTimeout(()=>{
        resolve('Success')
      },2000)
      // reject('err')
    })
}

async function test(){
  returnPromise().then(
    resolve=>{console.log(resolve)},
    reject=>{console.log(reject)}
  )
}

test()
console.log('I COME HERE FIRST') // Executed first before promise is returned

Here's a simple example of how you could implement rejection handling(for reference only)

function gsrun() {
    return new Promise((resolve, reject) => {
      //resolve('Success')
       reject('err')
    })
}

async function authorizeCallback(){
    gsrun().then(
    resolve=>{finalData(resolve)},
    reject=>{console.log(reject)} // do error handling
  )
}

I know the order the functions execute, but I don't know how to solve this issue. What should I await or what should I change? — , Mar 10 '20 at 09:02
Think Felix King did a better job at scoping the problem clearly. Nonetheless to answer your question, by relocating your "return returnData;" into finalData() function actually solves the problem. This is so that once your gsrun() returns the Promise, finalData() is executed. Oh and I did not realize you edited your post for the full code. Sorry for posting an answer with reference to the previous code. — CodeCodey, Mar 10 '20 at 09:24

Felix Kling · Accepted Answer · 2020-03-10T09:11:45.320

You are still trying to return before the data was assigned. You are not "waiting" until client.authorize has called the callback because you can't. You can only "return data from a callback" if the callback is called synchronously. But that's not the case here. It doesn't matter that you declared the callback as async, what matters is how/when the caller (i.e. client.authorize) calls the callback.

Wrap the client.authorize in a promise and await that in your getData function.

async function gsrun(client) {
  const gsapi = google.sheets({
    version: 'v4',
    auth: client
  })

  const options = {
    spreadsheetId: '1d3ZiP1I9jJ2ddlD1Hx2ylWn1VFD_5lYQ9Ps9e9gEqI',
    range: 'Sheet1!A1:H5'
  }

  const data = await gsapi.spreadsheets.values.get(options)

  return data.data.values
}


function authorize(client) {
  return new Promise((resolve, reject) => {
    client.authorize((err, tokens) => {
      if (err) {
        reject(err);
      } else {
        resolve(tokens);
      }
    });
  });
}


async function getData() {
  const client = new google.auth.JWT(
    keys.client_email,
    null,
    keys.private_key, ['https://www.googleapis.com/auth/spreadsheets']
  )

  await authorize(client);
  return await gsrun(client);
}

Here authorize will throw an error if the authentication failed and return the tokens if it is successful.

I wrote a post about callbacks and data. While it doesn't go into promises, maybe it can help your understanding of callbacks.

How can I await data from a callback function and then return it?

2 Answers2