Chainable promises for node

In node version 0.1.30 the previous built-in promises where removed. This library gives you the option of adding in a very similar functionality.

A promise can either wrap an arbitrary operation, or it can be used as a stand-alone "promise". These promises also have the ability to be chained -- executing multiple (asynchronous or synchronous) operations in a sequential fashion, breaking the chain on any error.

Usage:

Simple:

promise(fs.open, filename)(function(err, fd){
  // Called when fs.open "returns"
})(function(err, fd){
  // Also called when fs.open "returns"
})

Chaining:

promise(fs.stat, __filename)
  .then(fs.cat, __filename)
  (function(err, data){
    if (err) throw err;
    else sys.puts('successfully read '+data.length+' bytes.');
  })

If an error occurs in the middle of the chain (e.g. at fs.stat), the chain is broken and the outer promise is closed (called) with the error.

To transform or operate on intermediate return values, you can pass a regular function to then:

promise(fs.stat, __filename)
  .then(function(stats){
    if (!stats.isFile())
      throw new Error('not a file');
    return promise(fs.cat, __filename)
  })
  (function(err, data){
    if (err) throw err;
    else sys.puts('successfully read '+data.length+' bytes.');
  })

Stand-alone promises and closing:

function myAsyncFunction(fn) {
  var p = promise();
  // at some point in time or right now:
  p.close(error, result);
  return p;
}

Promises can be passed on through your code where arbitrary machinery might add callbacks and/or "links to the chain".

MIT license

Copyright (c) 2010 Rasmus Andersson

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.