How to await a callback function call in Node.js?

Question

I am new to Node.js and Javascript, I used npm package retry to send requests to server.

const retry = require('retry');

async function HandleReq() {

//Some code
return await SendReqToServer();
}

async function SendReqToServer() {
 
operation.attempt(async (currentAttempt) =>{
        try {
            let resp = await axios.post("http://localhost:5000/api/", data, options);
            return resp.data;
        } catch (e) {
            if(operation.retry(e)) {throw e;}
        }
    });
}

I get empty response because SendReqToServer returns a promise before function passed to operation.attempt resolves the promise.

How to solve this issue ?

SendReqToServer doesn't return anything, is this on purpose? — evolutionxbox
– evolutionxbox, Commented Mar 4, 2021 at 14:01
return resp.data returns data to the .attempt callback. You'll want to return operation.attempt(...) to return the value to SendReqToServer. — Phoenix
– Phoenix, Commented Mar 4, 2021 at 14:03
@ThomasSablik Yes, but it resolves immediately after calling operation.attempt() - without actually waiting for anything - since there's basically an implicit return undefined; at the end of the function. — Nikolaj Dam Larsen
– Nikolaj Dam Larsen, Commented Mar 4, 2021 at 14:09
@ThomasSablik yes indeed. - OP: SendReqToServer resolves to undefined, I assume this is not on purpose — evolutionxbox
– evolutionxbox, Commented Mar 4, 2021 at 14:09

Thomas Sablik · Accepted Answer · 2021-03-04 14:23:35Z

3

The solution to this question depends on operation.attempt. If it returns a promise you can simply return that promise in SendReqToServer, too. But usually async functions with callbacks don't return promises. Create your own promise:

const retry = require('retry');

async function HandleReq() {

//Some code
return await SendReqToServer();
}

async function SendReqToServer() {
 
    return new Promise((resolve, reject) => {
        operation.attempt(async (currentAttempt) => {
            try {
                let resp = await axios.post("http://localhost:5000/api/", data, options);
                resolve(resp.data);
                return resp.data;
            } catch (e) {
                if(operation.retry(e)) {throw e;}
            }
        });
    });
}

edited Mar 4, 2021 at 14:23

answered Mar 4, 2021 at 14:14

Thomas Sablik

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6 Comments

Ajay Sabarish Over a year ago

thanks a lot, this works, but the above mentioned issue happens because the callback function passed waits for axios's promise to resolve, but operation.attempt's doesn't, is my understanding correct ?

Thomas Sablik Over a year ago

@AjaySabarish That's correct. operation.attempt is called and it almost immediately finishes. The callback runs asynchronously.

Ajay Sabarish Over a year ago

Thanks, but you said if operation.attempt returns a promise, we can return it as such, but how can we be sure that that promise will contain the value returned in it's callback's promise ?

Thomas Sablik Over a year ago

@AjaySabarish You have to read documentation or source code to be see what it actually returns.

Ajay Sabarish Over a year ago

oh ok, got it, thanks, but it need not be always the case right, even if operation.attempt returns a promise ?

|

nodeDevcoder · Accepted Answer · 2021-03-04 14:06:43Z

0

Returning operation.attempt() will return resp.data if there were no errors in the function to sendReqToServer(). Currently, you're just returning the resp.data to operation.attempt(). You need to also return operation.attempt()

const retry = require('retry');

async function HandleReq() {

//Some code
return SendReqToServer();
}

async function SendReqToServer() {
 
return operation.attempt(async (currentAttempt) => {
        try {
            let resp = await axios.post("http://localhost:5000/api/", data, options);
            return resp.data;
        } catch (e) {
            if(operation.retry(e)) {throw e;}
        }
    });
}

answered Mar 4, 2021 at 14:06

nodeDevcoder

4973 silver badges14 bronze badges

2 Comments

Thomas Sablik Over a year ago

What if operation.attempt doesn't return a promise? This won't work.

Ajay Sabarish Over a year ago

Thanks a lot for responding. operation.attempt doesn't return a promise

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