CODEWITHSUNDEEP
javascriptnode.jsasync-awaites6-promise
Seneca
Seneca

Reputation: 2412

node callback to promise with async/await

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?

Upvotes: 7

Views: 7036

Answers (2)

aegyed
aegyed

Reputation: 1330

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

Upvotes: 10

Bergi
Bergi

Reputation: 664528

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?

Upvotes: 4

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