Can't promisify callback based function

Question

I want to use the library astro-js where a typical call in their docs looks like this:

const aztroJs = require("aztro-js");

//Get all horoscope i.e. today's, yesterday's and tomorrow's horoscope
aztroJs.getAllHoroscope(sign, function(res) {
   console.log(res);
});

For several reasons, I would like to use it using async/await style and leverage try/catch. So I tried promisify like this:

const aztroJs = require("aztro-js");
const {promisify} = require('util');
const getAllHoroscopeAsync = promisify(aztroJs.getAllHoroscope);

async function handle() {
  let result, sign = 'libra';
  try {
    result = await getAllHoroscopeAsync(sign);
  }
  catch (err) {
    console.log(err);
  }
  console.log("Result: " + result);
}

However, when I log result it comes as undefined. I know the call worked since the library is automatically logging a response via console.log and I see a proper response in the logs.

How can I "await" on this call? (even by other means if this one is not "promisifyable")

`promisify` expects a callback where the first argument is an/the error. — tkausl, Aug 21 '19 at 18:34
a) make sure to get the `this` context of the method right b) make sure that the method complies with the nodeback convention. — Bergi, Aug 21 '19 at 18:50
`const getAllHoroscopeAsync = promisify((sign, cb) => { aztroJs.getAllHoroscope(sign, res => { cb(null, res); }); });` should work. — Patrick Roberts, Aug 21 '19 at 18:55
your code seems to be ok..How are you calling your handle method?? — Mohammed Amir Ansari, Aug 21 '19 at 18:57

score 4 · Accepted Answer · answered Aug 21 '19 at 23:08

util.promisify() expects the callback function to accept two arguments, the first is an error that must be null when there is no error and non-null when there is an error and the second is the value (if no error). It will only properly promisify a function if the callback follows that specific rule.

To work around that, you will have to manually promisify your function.

// manually promisify
aztroJs.getAllHoroscopePromise = function(sign) {
    return new Promise(resolve => {
        aztroJs.getAllHoroscope(sign, function(data) {
            resolve(data);
        });
    });
};

// usage
aztroJs.getAllHoroscopePromise(sign).then(results => {
    console.log(results);
});

Note, it's unusual for an asynchronous function that returns data not to have a means of returning errors so the aztroJs.getAllHoroscope() interface seems a little suspect in that regard.

In fact, if you look at the code for this function, you can see that it is making a network request using the request() library and then trying to throw in the async callback when errors. That's a completely flawed design since you (as the caller) can't catch exceptions thrown asynchronously. So, this package has no reasonable way of communicating back errors. It is designed poorly.

score 2 · Answer 2 · answered Apr 28 '20 at 08:03

Try custom promisified function

aztroJs.getAllHoroscope[util.promisify.custom] = (sign) => {
  return new Promise((resolve, reject) => {
    aztroJs.getAllHoroscope(sign, resolve);
  });
};

const getAllHoroscopeAsync = util.promisify(aztroJs.getAllHoroscope);

score 1 · Answer 3 · answered Aug 21 '19 at 19:00

1

You could change your getAllHoroscopeAsync to a promise function

Example:

const getAllHoroscopeAsync = (sign) => 
    new Promise(resolve => 
            aztroJs.getAllHoroscope(sign, (res) => resolve(res)));

answered Aug 21 '19 at 19:00

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Can't promisify callback based function

3 Answers3