donify

The opposite of promisify - convert a promise into a done callback

Usage

const donify = require('donify');

const myFunction = function(flag, done) {
    var promise = new Promise((resolve, reject) => {
        if(flag) {
            return resolve('Success');
        }
        return reject('Failure');
    });

    return donify(promise, done);
};

// If done is not defined, it returns the underlying promise
var promiseSuccess = myFunction(true);
promiseSuccess.then((result) => {
    // result is 'Success'
});

var promiseSuccess = myFunction(false);
promiseSuccess.catch((err) => {
    // err is 'Failure'
});

// If you provide a done function, it gets called instead
var doneSuccess = myFunction(true, (err, result) => {
    // err is null, result is 'Success'
});
// doneSuccess is true

var doneFailure = myFunction(false, (err, result) => {
    // err is 'Failure', result is undefined
});
// doneFailure is true

Installation

Install with npm:

$ npm install donify

Why

I can feel you judging this project, saying "For all things holy, why would you ever want to do this?!?!?!" As it turns out, when refactoring a huge project from callbacks to Promises, this is really helpful. By donifying your functions, you can incrementally convert from callbacks to Promises without having to do it all in one go. For example, say you have this legacy function:

const myLegacyFunction = function(params, done) {
    someCallbackFunction(params, (err, results) => {
        done(err, results);
    });
};

You can easily rewrite it as:

const donify = require('donify');
const {promisify} = require('util');

const myLegacyFunction = function(params, done) {
    var promise = promisify(someCallbackFunction)(params);

    return donify(promise, done);
};

This retains 100% backward compatibility for your existing code but allows you to start refactoring to promises by eliminating the done callbacks as needed. It also allows you to have older, callback-based packages seamlessly call your Promise based functions.

I found myself writing this code over and over, so decided to simply pull it into a package with all the tests and stuff.

License

Apache 2.0