Promise patterns

Promises are great for giving us a way of writing asynchronous code without having to indent our code but if that's the only thing you're using them for then you're missing the point of promises. Promises are an abstraction that make doing several things easier. They have two properties that make them easier to work with:

1. You can attach more than one callback to a single promise
2. values and states (errors) get passed along

Because of these properties it makes common asynchronous patterns using callbacks easy. Here are some cases which may pop up from time to time:

NOTE: For the below I'm going to use APlus which is an implementation of the Promises/A+ spec that we walked through the development of in Promises/A+ - Understanding the spec through implementation. Some of the code below will also be available in the repository under aplus.extras.js. Also we will be using the "Node way" of defining async functions where the first argument takes a function to be run on error and the second function takes the success callback.

Converting callback functions to return promises

We don't want to rewrite all our already written functions to return promises and there are lots of useful libraries out there that already use callbacks. In fact we probably don't want to write and functions that we may share in external libraries later to use promises either. This is because there is no native implementation of promises at the moment. Because of the spec, promises are largely compatible but if we're using one implementation for the library the user might be using a different one for their code. Instead it's better to keep using callbacks for the definition of asynchronous functions as they're the base building block and let the user convert them to use promises if they wish. This can easily be acheived and below is an example of how you can convert a node.js style callback function to use the Aplus implementation of promises:

// take a callback function and change it to return a promise
Aplus.toPromise = function(fn) {
 return function() {

  // promise to return
  var promise = Aplus();

  //on error we want to reject the promise
  var errorFn = function(data) {
   promise.reject(data);
  };
  // fulfill on success
  var successFn = function(data) {
   promise.fulfill(data);
  };

  // run original function with the error and success functions
  // that will set the promise state when done
  fn.apply(this,
   [errorFn, successFn].concat([].slice.call(arguments, 0)));

  return promise;
 };
};

Sometimes a library will already have functions that have their own callback chains setup. In this case you only have to wrap the top function if that is all you need.

Sequential Calls

This is where promises excel. As the return value of a "then" is a promise that gets fulfilled with the value returned by the given function we only have to chain together to get the next value. That's great if your function can return a value directly but if it's asynchronous (which is the whole reason you're using a promise in the first place)  then you'll want to return a promise which then gets used as the basis for firing the next chained "then" method.

an example of promises:

var asyncAddOne = Aplus.toPromise(function(err, cb, val) {
 setTimeout(function() {
  cb(val + 1);
 });
});

var asyncMultiplyTwo = APlus.toPromise(function(err, cb, val) {
 setTimeout(function() {
  cb(val * 2);
 });
});

var asyncInverse = APlus.toPromise(function(err, cb, val) {
 setTimeout(function() {
  if (val === 0) {
   return err('value is zero');
  }
  cb(1 / val);
 });
});

var alertResult = function(value) {
 alert(value);
};

asyncAddOne(1)
 .then(asyncMultiplyTwo)
 .then(asyncInverse)
 .then(alertResult);

You can see that I wrote the asynchronous methods using normal callback style and converted them to return promises. You could Instead write them directly to use a promise like this:

var asyncAddOne = function(val) {
 var promise = APlus();
 setTimeout(function() {
  promise.fulfill(val + 1);
 });
 return promise;
};

Error Handling

If you are calling several services and want any error to short circuit then promises excel at this. If doing this with callbacks we only have to declare each function in turn and put our single error handling function at the end. In the previous example we have an inverse function that can call an error function, if we wanted to write this as a promise directly we could have just thrown an error like so:

var asyncInverse = function(val) {
 var promise = APlus();
 if (val === 0) {
  promise.reject('can not inverse zero');
 }
 setTimeout(function() { 
  APlus.fulfill(1 / val);
 });
 return promise;
};

Though we also have the option of just throwing an error instead which will pass along the value as the error thrown. To add a single error handling function we just need to tack it on to the end of our call:

var onError = function(err) {
 console.error(err);
};

asyncAddOne(1)
 .then(asyncMultiplyTwo)
 .then(asyncInverse)
 .then(alertResult)
 .then(undefined, onError);

Note how the first argument is undefined as the second argument is used for the error. We could just have easily put the onError with the alertResult call, but then we wouldn't catch any error from the alertResult function.

Pool

Sometimes you want the result of several operations before you continue. Instead of waiting for each one to finish before going on with the next we can save some time by firing them all off at once. Basically we have to catch when a promise is finished (through both it's success and error callbacks) and save it's value to an array. Once they're all done then we can fulfill or reject the promise with the given values. We'll first need some code that can do this for us - here is a method that we can use:

// resolve all given promises to a single promise
Aplus.pool = function() {

 // get promises
 var promises = [].slice.call(arguments, 0);
 var state = 1;
 var values = new Array(promises.length);
 var toGo = promises.length;

 // promise to return
 var promise = Aplus();

 // whenever a promise completes
 var checkFinished = function() {
  // check if all the promises have returned
  if (toGo) {
   return;
  }
  // set the state with all values if all are complete
  promise.changeState(state, values);
 };

 // whenever a promise finishes check to see if they're all finished
 for (var i = 0; i < promises.length; i++) {
  (function(index) {
   promises[index].then(function(value) {
    // on success
    values[index] = value;
    toGo--;
    checkFinished();
   }, function(value) {
    // on error
    values[index] = value;
    toGo--;
    // set error state
    state = 2;
    checkFinished();
   });
  })(i);
 };

 // promise at the end
 return promise;
};

Now all we need to do is pass in all the promises we need:

Aplus.pool(
 getName(),
 getAddress()
).then(function(value) {
 var name = value[0];
 var address = value[1];
 alert(name + ' lives at ' + address);
}, function() {
 alert('unable to retrieve details');
});

In the above getName and getAddress both return promises - though it is possible to tweak the pool function logic so that any non-promise return value is just passed through directly. There are some implementations that do this such as jQuery's $.when

Some fun

Okay, there are some common patterns. Let's see if we can build on top of those. Let's say I'm racing some functions. The fastest to return wins the race. They also may error. Let's write such a function that will return a promise that is only fulfilled or rejected after a timeout:

var racer = function(err, success, value) {
 setTimeout(function() {
  if (Math.random() < 0.05) {
   err(value);
  } else {
   success(value);
  }
 }, Math.random() * 2000);
};

var promiseRacer = Aplus.toPromise(racer);

We've given it a 5% chance of error (or if you're like me and pretending it's a horse race - breaking it's leg and not finishing). Now we need to write a function that will run them all:

// return the value of the first succesful promise
Aplus.first = function() {

 // get all promises
 var promises = [].slice.call(arguments, 0);

 // promise to return
 var promise = Aplus();

 // if all promises error out then we want to return an error
 Aplus.pool.apply(Aplus, promises).then(undefined, function(value) {
  promise.reject(value);
 });

 // when there is a success we want to fulfill the promise
 var success = function(value) {
  promise.fulfill(value);
 };

 // listen for success on all promises
 for (var i = 0; i < promises.length; i++) {
  promises[i].then(success);
 }

 return promise;
};

I simple have to hook up the "then" success function to a single promise, as it can only be fulfilled once. You'll also see that I'm using Aplus.pool as well. This is for error handling, we need to handle the case when all the "horses" break their leg. Now let's race them!

Aplus.first(
 promiseRacer('Binky'),
 promiseRacer('Phar Lap'),
 promiseRacer('Sea Biscuit'),
 promiseRacer('Octagonal'),
 promiseRacer('My Little Pony'),
).then(function(value) {
 alert(value + ' wins!');
}, function() {
 alert('no function finished');
});

And there you have it, a simple racing game made with promises.