Promises and Q

promises?

A promise represents the result of an asynchronous operation

It exists in three states:

Pending
Fulfilled
Rejected

So, it's similar to an event-listener...but

A promise can only succeed or fail once
If a promise has succeeded or failed and there is a callback, that callback will be called

THEN

"Promise me something then...."

Interaction with a promise takes place through its then method, which registers those callbacks to handle the promise’s eventual value or the reason why the promise cannot be fulfilled.

WHY USE PRomises?

To avoid the notorious callback hell

doSomething.method("user", "pass", {
  success: function(user) {
    query.find({
      success: function(results) {
        results.save({ key: value }, {
          success: function(result) {
            // the object was saved.
          }
        });
      }
    });
  }
});

Error handling:
promises propagate errors,

not calling any callback until an error handler is encountered

A basic promise with q


var promise = function(err, result) { 
    var deferred = Q.defer(); 
    if (err) { 
        deferred.reject(new Error(err)); 
    } else { 
        deferred.resolve(result); 
    } 
    return deferred.promise;
};

So, the promise becomes an object that is thenable

...so do something with the result

promise.then(function(result) { 
    doSomething(result) 
});

chaining

promise.then(function(result) { 
      return getSomething(result); 
    }).then(function(gotten) {
      return doSomething(gotten);
    }).fail(function(error) {
       errorHandle(error);
    }).done();

EXAMPLE

function statusCode(url) {
    var deferred = Q.defer();
    request.head(url, function(err, response, body) {
        if (err) deferred.resolve(err);
        deferred.resolve(response);
    });
    return deferred.promise
};

OR Similarly via Q.nfcall

function statusCode() {
   return Q.nfcall(request.head, url)
};

statusCode().then(function(data) {
    console.log(data.statusCode) \\ returns 200
};

Other libraries

Bluebird

RSVP

when.js

list goes on....