node callback to promise with async/await

Question

I'm trying to write a simple function that converts node-style callback functions to promises, so I can use them with async/await.

Current code:

function toPromise(ctx, func, ...args) {
  let newPromise;

  args.push((err, res) => {
    newPromise = new Promise((resolve, reject)=> {
       if(err) reject(err);
       else{
        resolve(res) 
      };
     });
    });

   func.apply(ctx, args);

   return newPromise;
}

example usage:

const match = await toPromise(user, user.comparePassword, password);
//trying to avoid the following:
user.comparePassword(password, (err, res) => {
     ... });

This probably doesn't make any sense with some great libraries out there, but I'm just trying to code this as an exercise.

Problem is of course match evaluates to undefined, and apparently the promise gets resolved after the await syntax line.

Any idea how I can resolve this issue?

Answer 1

Since node v8.0.0 they added util.promisify.

const util = require('util');
const fs = require('fs');

const stat = util.promisify(fs.stat);
stat('.').then((stats) => {
  // Do something with `stats`
}).catch((error) => {
  // Handle the error.
});

Ref: https://nodejs.org/api/util.html#util_util_promisify_original

Answer 2

Your problem is that you are constructing the newPromise inside of the asynchronous callback. So you've still got undefined when you return it. Instead, you will need to call the Promise constructor immediately, and only put the resolve/reject in the asynchronous callback:

function toPromise(ctx, func, ...args) {
    return new Promise((resolve, reject) => {
        args.push((err, res) => {
            if (err) reject(err);
            else resolve(res);
        });
        func.apply(ctx, args);
    });
}

See also How do I convert an existing callback API to promises?