Error-first callback style Futures.

error-first-futures-examples.js

var log = console.log.bind(console);

// Like .then(), except we move to a single callback
asyncAPI().when(function(err, value) {
  if (err) {
    // error handling/recovery
    // ...
    // If return is instanceof Error, continues error chain
    return new Error(err.toString()); 
  }
  return "success!";
}).when(log);

// Like .done(), except single callback
asyncAPI().finished(log);