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Callback to Promise converter. A Proposal is a bridge function between node-style asynchronous functions with callbacks in the form of (err, data) => void or (err, [data]) => void (which from here on out I'll refer to as nodebacks) and ECMAScript 6 Promises.


Required: nodejs (tested against v0.10 and 0.12), or io.js (tested against v1.4.2), npm.

npm i proposal --save


Proposal(nodeback[, args]) - takes a nodeback and converts it into a Promise.

If arguments are supplied, the nodeback is executed and a Promise is returned. Use it like any other Promise you've used before, with .then() and .catch().

If no arguments are supplied, Proposal will return a function that, when executed with its parameters, will then return a Promise. This is useful if, for example, you want to execute that function multiple times to pass in different arguments.


1. Create a Proposal

Create a Proposal function by calling Proposal() with 1 argument: the function you'd like to convert.

var fs = require('fs'),
  Proposal = require('proposal'),
  readProposal = Proposal(fs.readFile);

At this point, readProposal is a function, that when invoked with fs.readFile's parameters, will return a Promise containing the result of the file read operation. We'll use this Proposal twice below, once to read the system's HOSTS file and again to read the system's Apache configuration file.

var path = require('path'),
  hostsFile = path.resolve('/etc/hosts'),
  apacheConfig = path.resolve('/etc/httpd/httpd.conf'),
  hostsRead = readProposal(hostsFile),
  apacheRead = readProposal(apacheConfig);
hostsRead.then(function (txt) {
  //do stuff with txt
.catch(function (err) {
  //handle the error
//apacheRead is also available as a Promise here

2. Create a Promise containing the result of a file read

You can skip the intermediate Proposal function and get a Promise directly by supplying the nodeback's arguments when invoking.

var fs = require('fs'),
  path = require('path'),
  Proposal = require('proposal'),
  filepath = path.resolve('data/example.json'),
  readFile = Proposal(fs.readFile, filepath);
  readFile.then(function (data) {
    //do stuff with data
  .catch(function (err) {
    //handle the error


  1. My nodeback doesn't take any arguments, just a callback. How can I get a Promise back instead of a Proposal?

    Just invoke the resulting Proposal. Example:

    var Proposal = require('proposal');
    function closeConnection(callback) {
      //this is an example of a nodeback with no arguments
      callback(err, data);
    //create the Proposal to close the connection
    var closeProposal = Proposal(closeConnection);
    //invoke the Proposal to return a Promise
    var closePromise = closeProposal();
    closePromise.then(function (data) {
      //do something with your data
    .catch(function (err) {
      //handle your error
  2. Does Proposal work with node crypto functions?

Yes! As you may or may not know, many of the functions in nodejs's crypto module, like randomBytes, are async when a callback is passed in but sync when the callback argument is omitted. Proposal will work with these and preserve asynchronicity. Example from the unit tests:

var buffy = Proposal(crypto.randomBytes, 512);
console.log(buffy instanceof Promise); // => true, is not a value
  1. My nodeback has several data return values, like child_process.exec, which, on success, returns an stdout and an stderr Buffer. Is this supported?

Yes, in this case resolve will return an Array with the data return values. While ugly-ish, it does work well in practice, as multiple arguments are not allowed in onFulfilled.

  1. I have another question that's not listed here.

    Raise an issue and I'll get to it as soon as I can. Thanks for reading this far!



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