Q.md

title	excerpt
Q	a library to create and manage promises

Q

Q is a promise library providing many more utilities than the native Promise implementation.

List of features:

• Deferred values (old school): Q.defer
• Multiple promises handling: Q.all, Q.any, Q.spread
• Promise creation, resolve and reject: Q(value), Q.when(value), Q.reject(value)
• Convert properties to promises: Q.get, Q.set
• Convert functions to promises: Q.fcall, Q.invoke
• Convert node-based callbacks to promises: Q.nfcall, Q.ninvoke
• Convert timeout functions: Q.timeout
• Support notify functions
• Support chaining and tapping

Real world example from a Node.js application:

// Traditional implementation
function authenticate (req, res, next) {
  User.findOne({ id: req.id }, function (err, user) {
    if (err) {
      return next('not found');
    }

    user.save(function (err, response) {
      // ...
    });
  });
}

// Q Based
function authenticate(req, res, next) {
  return Q(req.id)
    .then(function (id) {
       return Q.nfcall(User, 'findOne', id);
    })
    .then(function (user) {
       return Q.ninvoke(user, 'save');
    })
    .then(function (saved) {
       return res.send(201);
    })
    .catch(function (err) {
      return next(err);
    });
}