prom2cb

An adapter library converting Node.js-style functions with callbacks into functions constructing promises and vice-versa.

It has only a few simple requirements. For "chaining" Node.js-style callbacks to promises, we require that promises implement a .then() method that conforms to the Promises/A+ standard (which should be obvious).

For wrapping Node.js-style functions into functions returing promises, we require a Promise prototype providing a constructor with the following structure: new Promise(function(resolve, reject) { ... }) in which the function parameters refer to callbacks that should be called when a promise gets fulfilled or rejected.

Installation

Currently, this library is tested for usage with Node.js and web browsers.

Node.js

Usage on Node.js is straight forward. It can be installed into a working directory with the NPM package manager by running:

$ npm install prom2cb

In the code, the module can be imported with:

var prom2cb = require('prom2cb');

Browser

For usage in the browser copy lib/prom2cb.js into a folder accessible by a web page. Then add the following script include to the HTML code of that web page:

<script type="text/javascript" src="prom2cb.js"></script>

Usage

This library provides two conversion functions.

Chaining a Node.js-style callback function to a promise

prom2cb.chainCallback() can be used to chain a Node.js-style callback (which first argument corresponds to an error object) to a promise. This function replaces the .then() function invocation, that is normally used to obtain the result of a promise.

With this function you can, for example, easily integrate promise-style function invocations with a Node.js-function style control flow abstraction function:

slasp.sequence([
    function(callback) {
        prom2cb.chainCallback(Task.sync(), callback);
    },
    
    function(callback) {
        prom2cb.chainCallback(Task.create({
            title: "Get some coffee",
            description: "Get some coffee ASAP"
        }), callback);
    },
    
    function(callback) {
        prom2cb.chainCallback(Task.create({
            title: "Drink coffee",
            description: "Because I need caffeine"
        }), callback);
    },
    
    function(callback) {
        prom2cb.chainCallback(Task.findAll(), callback);
    },
    
    function(callback, tasks) {
        for(var i = 0; i < tasks.length; i++) {
            var task = tasks[i];
            console.log(task.title + ": "+ task.description);
        }
    }
], function(err) {
    if(err) {
        console.log("An error occured: "+err);
        process.exit(1);
    } else {
        process.exit(0);
    }
});

Converting a Node.js-style function into a function returning a promise

prom2cb.promisify() can be used wrap a Node.js-style function, in which the last function parameter corresponds to a callback, into a function returning a promise. The wrapped function takes the same function parameters (minus the callback).

Because the wrapped functions return promises, we can "chain" them to other promises through .then() function invocations:

var fs = require('fs');
var Promise = require('rsvp').Promise; // We use RSVP to construct promises

/* Wrap the fs.readFile function into a function return a promise */
var readFile = prom2cb.promisify(Promise, fs.readFile); 

/* Invoke the function as a promise */
readFile("hello.txt").then(function(data) {
    console.log("File contents is: "+data);
}, function(err) {
    console.log("Error opening file: "+err);
});

Instead of defining and adapting the function first and invoking it later, we can also combine these aspects into a one liner:

prom2cb.promisify(Promise, fs.readFile)("hello.txt").then(function(data) {
    console.log("File contents is: "+data);
}, function(err) {
    console.log("Error opening file: "+err);
});

The latter is typically useful if you have to invoke a function only once.