Promises are a tool to help write asynchronous code in a more readable style that looks more like synchronous code.
In short, promises allow you to interact with a value that may or may not be available yet.
Promises let wrap, and even chain, asynchronous operations using a consistent API, avoiding writing nested anonymous callbacks (the "pyramid of doom"). And they let you handle any errors that happen during those operations.
The Y.Promise class is compatible with the
Promises/A+
specification.
To include the source files for Promise and its dependencies, first load the YUI seed file if you haven't already loaded it.
<script src="http://yui.yahooapis.com/3.18.1/build/yui/yui-min.js"></script>
Next, create a new YUI instance for your application and populate it with the
modules you need by specifying them as arguments to the YUI().use() method.
YUI will automatically load any dependencies required by the modules you
specify.
<script>
// Create a new YUI instance and populate it with the required modules.
YUI().use('promise', function (Y) {
// Promise is available and ready for use. Add implementation
// code here.
});
</script>
For more information on creating YUI instances and on the
use() method, see the
documentation for the YUI Global Object.
As mentioned above, promises allow you to interact with a value that may or may not be available yet. In synchronous code, values are assigned to variables and immediately available to use, but if you need to use or assign a value that depends on an asynchronous operation to get, the rest of your code needs to be wrapped in a callback that is executed when that asynchronous operations completes.
Callbacks work, but they don't maintain any state, the APIs responsible for the callbacks are likely to differ, and they might not handle errors. It's also quite easy to find yourself building up multi-step transactions by nesting anonymous callbacks multiple levels deep.
Promises address this by providing an object that can be referred to immediately and any time in the future that represents the value produced by the asynchronous operation. Here's how you use them:
Promises operate using two methods: the Y.Promise constructor, and the
promise instance's then() method.
// Create a promise for a value
var promise = new Y.Promise(function (resolve, reject) {
var promisedValue;
var reasonForFailure = new Error('something bad happened');
// ...do some work to assign promisedValue, most likely asynchronously
// When the work is done, fulfill the promise with the resolve function,
// which was passed in the arguments.
resolve(promisedValue);
// Or if something went wrong, reject the promise with the reject function,
// also passed in the arguments.
reject(reasonForFailure);
});
// Do something with the promised value using the then() method. then() takes
// two functions as arguments. promise.then(onFulfilled, onRejected);
promise.then(
// aka onFulfilled
function (promisedValue) {
alert("Here's that value I promised I'd get for you: " + promisedValue);
},
// aka onRejected
function (reason) {
alert("Oh no! I broke my promise. Here's why: " + reason);
});
The Y.Promise constructor takes as its argument a function we'll call the
"executor function". This function is responsible for saying when the
promised value is ready, or notifying that something went wrong.
The executor function receives two customized functions as its arguments,
commonly called resolve and reject. If the work in the executor
function to get the promised value completes successfully, pass the value
to the resolve() method. If something went wrong, pass the
reason—commonly an Error—to the reject() method.
// dataPromise represents the data parsed from the IO response,
// or the error that occurred fetching it
var dataPromise = new Y.Promise(function (resolve, reject) {
Y.io('getdata.php', {
on: {
success: function (id, response) {
// The IO completed, so the promise can be resolved
try {
resolve(Y.JSON.parse(response.responseText));
} catch (e) {
// any failure to produce the value is a rejection
reject(e);
}
},
failure: function (id, response) {
// The IO failed
reject(new Error("getdata.php request failed: " + response));
}
}
});
});
Promises can be in one of three states:
pending - the promised value is not ready yet (default)fulfilled - the value is readyrejected - something went wrong, the value can't be produced
"Resolving" a promise moves a pending promise to either fulfilled or
rejected, though the term is often used interchangeably with "fulfill"
(it's good to have a positive outlook). Once a promise is fulfilled or
rejected, it can't be transitioned to another state.
There are two ways promises get resolved. The first is using the
resolve() function passed to the executor function in the Y.Promise
constructor. We'll talk about the second way when we discuss promise chaining.
Since the promised value probably isn't ready when you create the promise,
you can't synchronously consume the value. Schedule the code that will use
the promised value to execute with the promise's then() method.
then() takes two callbacks as arguments, that we call onFulfilled and
onRejected. As you might have guessed, the onFulfilled callback is
executed if the promise resolves to a value, and the onRejected callback
is executed if it is rejected.
Only one of the callbacks will be executed, and only once.
Both callbacks are
optional, though in practice you'll always pass at least one to
then().
var stuff;
var promise = new Y.Promise(getStuff);
// When getStuff says the promise is fulfilled, update the stuff variable.
// No onRejected callback is passed, so if there was an error, do nothing.
promise.then(function (stuffValue) {
stuff = stuffValue;
});
// Stuff isn't populated yet because the promise hasn't been fulfilled
console.log("Stuff value is " + stuff); // => "Stuff value is undefined"
You can call then() on the promise as many times as you like. The same
value will be passed to each then() callback.
It's important to note that even if the getStuff function above resolved
the promise immediately, callbacks scheduled with then will
always be called asynchronously. So the example code above
will always log "Stuff value is undefined", regardless of whether
getStuff operates synchronously or asynchronously.
To limit the runtime impact of then callbacks always being executed
asynchronously, they are scheduled using
Y.soon(), which
will attempt to avoid any minimum delay that some browsers impose on
setTimeout.
Here's where things start getting fun. When you call promise.then(...),
a new promise is returned. The new promise will resolve when either of the
original promise's onFulfilled or onRejected functions returns a value
or throws an error. This allows you to schedule several operations using
chained then() calls.
// Verbose form
startSpinner();
// Create the initial promise
var userDataLoaded = new Y.Promise(function (resolve, reject) {
Y.io('getUserData.php', {
data: 'id=1234',
on: {
success: function (id, response) {
try {
resolve(Y.JSON.parse(response.responseText));
} catch (e) {
reject(e);
}
},
failure: function (id, response) {
reject(new Error(response));
}
}
});
});
// after the user data is loaded, render stuff or show the loading error
var uiUpdated = userDataLoaded.then(renderTemplates, showError);
// after the UI is updated, stop the spinner
uiUpdated.then(stopSpinner);
// Concise form (more common)
// Note Y.Promise can be called without 'new'
Y.Promise(function (resolve, reject) { Y.io(...); })
.then(renderTemplates, showError) // returns another promise
.then(stopSpinner); // returns another promise
A chained promise is resolved by the return value of the previous promise's callbacks. Or, if an error is thrown, the chained promise is rejected.
Note that functions will return undefined if no explicit return
statement is included. That will result in the promise being fulfilled with
a value of undefined. Sometimes that's okay, but it's often helpful to pass
along some data.
function renderTemplates(userData) {
// Update the UI
Y.one('#userForm').setHTML(Y.Lang.sub(MyApp.userFormTemplate, userData));
// return a value to resolve the chained promise (aka uiUpdated) and pass
// to the uiUpdated's then(onFulfilled) callback, stopSpinner
return true;
}
function stopSpinner(updated) {
// updated will receive the return value of the previous promise's callback
// In this case, the boolean true.
var face = updated ? happyFace : sadFace;
spinnerNode.replace(face).hide(true);
}
// Using the original promise from the example above
userDataLoaded
.then(renderTemplates, showError)
.then(stopSpinner);
When a promise is rejected, the onRejected callback (the second argument
to then()) is executed. Like onFulfilled, it is called with whatever is
passed to the executor function's reject() function.
The onRejected callback can then re-throw the error to propagate the
failed state, or recover from the failure by returning a value. Again,
without an explicit throw or return, the callback will return
undefined which will mark the failure as recovered, but with a resolved
value of undefined. This may not be what you want!
function showError(reason) {
Y.one('#userForm').hide(true);
Y.one('#message .details').setHTML(reason.message || reason);
Y.one('#message').show();
// Choosing not to re-throw the error, but consider it recovered from for
// the sake of this transaction. Returning false as resolved value to send
// to stopSpinner.
return false;
}
userDataLoaded
.then(renderTemplates, showError)
.then(stopSpinner);
Because showError returned a value, and didn't re-throw the reason,
the promise wrapping renderTemplates and showError was resolved to a
"fulfilled" state with a value of false. Since the promise was fulfilled,
not rejected, that promise's onFulfilled callback (stopSpinner) is
called with the value false.
Because thrown errors are caught by the Y.Promise internals and used as
a signal to reject a promise, it's possible to write promise chains that
fail silently. This can be hard to debug.
To avoid this, it's highly recommended to always include
an onRejected callback at the end of your promise chains. The reason you
only need to put one at the end is discussed below.
onFulfilled or onRejected
Both onFulfilled and onRejected callbacks are optional, though in
practice, you will always pass at least one. When a callback isn't provided
for a then() call in a promise chain, that promise is automatically
fulfilled with the value returned from the prior onFulfilled callback or
rejected with the reason thrown from the prior onRejected callback.
getHandleFromServerA()
.then(null, getHandleFromServerB)
.then(getUserData)
.then(renderTemplates, showError);
// Same code, commented
// Try to get a DB handle from Server A...
getHandleFromServerA()
// if that fails, try Server B, otherwise, pass through the Server A handle
.then(null, getHandleFromServerB)
// if either server provided a handle, get user data.
// otherwise, there was an error, so pass it along the chain
.then(getUserData)
// render the user data if everything worked.
// if there was an error getting a DB handle or getting user data show it
.then(renderTemplates, showError);
It's not uncommon to see promise chains with only onFulfilled callbacks,
then an onRejected callback at the very end.
As mentioned above, the return value from either onFulfilled or
onRejected fulfills the promise with that value. There is one
exception.
If you return a promise instead of a regular value (call it
returnedPromise), the original promise will wait for returnedPromise to
resolve, and adopt its state when it does. So if returnedPromise is
fulfilled, the original promise is fulfilled with the same value, and if
returnedPromise is rejected, the original promise is rejected with the
same reason.
Y.Promise(function (resolve, reject) {
Y.io('getDataUrl.php', {
on: {
success: function (id, response) {
resolve(response.responseText);
},
failure: function () {
reject(new Error("Can't reach the server"));
}
}
});
})
// Chain another async operation by returning a promise.
// Don't worry, we'll wait for you.
.then(function (data) {
return new Y.Promise(function (resolve, reject) {
// Do another async operation
Y.jsonp(data.url, {
on: {
success: resolve,
failure: reject
}
});
});
})
// Called after both async operations have completed. The data response
// from the JSONP call is passed to renderTemplates
.then(renderTemplates)
// Then wait for 2 seconds before continuing the chain
.then(function () {
return new Y.Promise(function (resolve) {
setTimeout(resolve, 2000);
});
})
.then(hideMessage, showError);
Similarly, you can pass a promise to the resolve() function passed to the
Y.Promise executor function.
Caution: Do not pass a promise to reject() or throw a
promise from a callback. You're definitely doing something wrong if you
find yourself doing that.
Y.when() For Promise Wrapping
If you're unsure if a variable has a value or a promise, or you want an API
to support both value or promise inputs, use Y.when(value) to wrap
non-promise values in promises. Wrapped non-promise values will be
immediately fulfilled with the wrapped value. Passing a promise to Y.when
will return the promise.
// Accept either a regular object or a promise to save
MyDatabase.prototype.save = function (key, data) {
// Ensure we are dealing with a promise and call then() to get its value
// return the promise chained off this then() call
return Y.when(data).then(function (data) {
// Store the data somehow, for instance in localStorage
localStorage.set(key, data);
});
};
Promise chaining works great to serialize synchronous and asynchronous
operations, but often several asynchronous operations can be performed
simultaneously. This is where Y.Promise.all() comes in.
Y.Promise.all() takes an array of promises and returns a new promise that
will resolve when all the promises in the array have resolved. The
resolved value will be an array of values from the individual promises, in
the order they were passed to Y.Promise.all().
If any one of the promises is rejected, the returned promise is immediately rejected with that reason, so failures can be dealt with sooner rather than later.
Y.Promise.all([
getUserAccountInfo(userId),
getUserPosts(userId, { page: 1, postsPerPage: 5 }),
getUserRank(userId)
])
.then(function (data) {
var account = data[0],
posts = data[1],
rank = data[2];
...
}, handleError);
Sometimes it is necessary to provide early feedback when doing several
transactions at the same time. It may be to send visual feedback to the user
in the browser or to chose the fastest service to respond in Node.js.
Promises provide this capability in Y.Promise.race().
Y.Promise.race() takes an array of promises and resolves to the first
promise that is fulfilled or rejected. Contrary to Y.Promise.all() which
waits for all promises to be resolved and fulfills to an array of all the
corresponding values, Y.Promise.race() fulfills to a single value, that of
the first promise in the list to resolve.
Y.Promise.race([
querySlowRemoteService(query),
getDataFromFastLocalCache(dataId)
])
.then(function (raceWinner) {
...
});
Y.Promise and...Events are used to create a relationship between two objects, and better represent an open communication channel. Promises represent single values, and chains encapsulate transactions.
It's not uncommon to have event subscribers launch a promise chain, or to have events fired from within operations inside a promise chain. They are complementary tools.
Y.AsyncQueue
Y.AsyncQueue is a tool for splitting up long synchronous operations into
asynchronous chunks to avoid blocking UI updates unnecessarily. It doesn't
(as yet) support asynchronous steps. It also supports conditional looping
and various other things that promises don't, out of the box.
Y.Parallel
Y.Parallel is similar to Y.Promise.all in that it provides a mechanism
to execute a callback when several independent asynchronous operations have
completed. However, it doesn't handle errors or guarantee asynchronous
callback execution. It is also transactional, but the batch of operations
is bound to a specific callback, where Y.Promise.all() returns a promise
that represents the aggregated values of those operations. The promise can
be used by multiple consumers if necessary.
There are a lot of opportunities inside YUI to move transactional APIs to consume and/or return promises rather than use callbacks or one-time events. While there are no set plans for which APIs will be changed or in what priority order, you can expect to see promises showing up across the library in the near future.