Promisebacker

Wraps an asynchronous function that either takes a callback or returns a promise, and allows it to do both.

Note: Promisebacker is still under development and not yet ready for production systems. Releases are stable but the API is subject to rapidly change.

Installation

npm install promisebacker

Example

var Promise = require('promise') // Any Promises/A+ compliant library will do.
  , promisebacker = require('promisebacker');
 
var takes_callback = function(arg1, arg2, callback) {
  // Does something...
  var error = false;
 
  // Asynchronously invokes callback.
  setTimeout(function() {
    if (error){
      return callback('uh oh!');
    }
    callback(null, 'hello world!');
  }, 3000);
};
 
var returns_promise = function(arg1, arg2) {
  // Does something...
  var error = false;
 
  // Returns promise that resolves asynchronously.
  return new Promise(function(resolve, reject) {
    setTimeout(function() {
      if (error) {
        return reject('uh oh!');
      }
      resolve('hello world!');
    }, 3000);
  });
};
 
var wrapped_callback = promisebacker(takes_callback);
 
// Now we can pretend this returns a promise...
wrapped_callback('alas', 'poor yorick')
  .then(function(result) {
    // result == 'hello world!'
  });
 
// ...or we can continue using it as a callback!
wrapped_callback('alas', 'poor yorick', function(err, result) {
  // err == null
  // result == 'hello world!'
});
 
// And we can do the same for functions that return promises.
var wrapped_promise = promisebacker(returns_promise);
 
wrapped_promise('alas', 'poor yorick')
  .then(function(result) {
    // result == 'hello world!'
  });
 
wrapped_promise('alas', 'poor yorick', function(err, result) {
  // err == null
  // result == 'hello world!'
});
 

Usage Notes

Promisebacker(Function target) assumes that you're trying to use a callback if the last argument passed is a Function of arity at least 2. If you want to force it to return promises, use Promisebacker.toPromise instead.

API Reference

We define a function to take a node-style callback (a nodeback) if it accepts a Function of arity at least 2 as its last argument and invokes that function whenever it finishes running. When invoking its callback, it must pass an error value as its first argument which must be truthy if and only if an error has occurred.

Promisebacker(Function target [, Object options]) -> Function

Wraps target such that it can either take a callback or return a promise.

• target must either return a promise or take nodebacks.
• options is an optional object with options.

If target takes nodebacks and calls its callback with multiple success values, the fulfillment value will be an array of them.

See the bluebird documentation for promisification for details.

Option: Object scope (default: N/A)

If you pass a scope, then target will have its this bound to scope (i.e. as if it were being called as scope.target).

Option: Boolean spread (default: false if nodeback is of arity at most 2, true otherwise)

Some nodebacks expect more than 1 success value but there is no mapping for this in the promise world. If spread is specified, the nodeback is called with multiple values when the promise fulfillment value is an array:

var example = Promisebacker(Promise.resolve);
example([1, 2, 3], function(err, result) {
  // err == null
  // result == [1, 2, 3]
});
 
var another = Promisebacker(Promise.resolve, {spread: true});
another([1, 2, 3], function(err, a, b, c) {
  // err == null
  // a == 1, b == 2, c == 3
});

Promisebacker.toPromise(Function target [, Object options]) -> Function

Same as above, but will always return a Promise even if the last argument is a Function of arity at least 2.