This is a brief introduction to using Promises in JavaScript, primarily aimed at frontend developers.
See here for a Chinese translation.
A Promise is a programming construct that can reduce some of the pains of asynchronous programming. Using Promises can help produce code that is leaner, easier to maintain, and easier to build on.
This lesson will mostly focus on ES6 Promise syntax, but will use Bluebird since it provides excellent error handling in the browser. The CommonJS syntax will need a bundler like browserify or webpack. See jam3-lesson-module-basics for an introduction to CommonJS and browserify.
To demonstrate, let's take the problem of loading images in the browser. The following shows an implementation using a Node style (error-first) callback:
function loadImage(url, callback) {
var image = new Image();
image.onload = function() {
callback(null, image);
};
image.onerror = function() {
callback(new Error('Could not load image at ' + url));
};
image.src = url;
}Tip: The above is implemented on npm as img.
Loading a single image is relatively easy, and looks like this:
loadImage('one.png', function(err, image) {
if (err) throw err;
console.log('Image loaded', image);
});However, as our application grows in complexity, so too does the code. If we were to take the same approach, but load three images, things get a little unwieldy:
loadImage('one.png', function(err, image1) {
if (err) throw err;
loadImage('two.png', function(err, image2) {
if (err) throw err;
loadImage('three.png', function(err, image3) {
if (err) throw err;
var images = [image1, image2, image3];
console.log('All images loaded', images);
});
});
});This tends to create a "Christmas Tree" of functions; and leads to code that is difficult to read and maintain. Further, if we wanted the images to load in parallel, it would need a more complex solution.
There are numerous abstractions built around the error-first callbacks, sometimes called "errbacks."
One way to solve the problem is with the async module:
var mapAsync = require('async').map;
var urls = [ 'one.png', 'two.png' ];
mapAsync(urls, loadImage, function(err, images) {
if (err) throw err;
console.log('All images loaded', images);
});Similar abstractions exist independently on npm, such as:
This approach is very powerful. It's a great fit for small modules as it does not introduce additional bloat or vendor lock-in, and does not have some of the other pitfalls of promises.
However, in a larger scope, promises can provide a unified and composable structure throughout your application. They will also lay the groundwork for ES7 async/await.
Let's re-implement the above with promises for our control flow. At first this may seem like more overhead, but the benefits will become clear shortly.
Below is how the image loading function would be implemented with promises. We'll call it loadImageAsync to distinguish it from the earlier example.
var Promise = require('bluebird')
function loadImageAsync(url) {
return new Promise(function(resolve, reject) {
var image = new Image();
image.onload = function() {
resolve(image);
};
image.onerror = function() {
reject(new Error('Could not load image at ' + url));
};
image.src = url;
});
}The function returns a new instance of Promise which is resolved to image if the load succeeds, or rejected with a new Error if it fails. In our case, we require('bluebird') for the Promise implementation.
The Promise constructor is typically only needed for edge cases like this, where we are converting a callback-style API into a promise-style API. In many cases it is preferable to use a promisify or denodeify utility which converts Node style (error-first) functions into their Promise counterpart.
For example, the above becomes very concise with our earlier loadImage function:
var Promise = require('bluebird');
var loadImageAsync = Promise.promisify(loadImage);Or with the img module:
var Promise = require('bluebird');
var loadImage = require('img');
var loadImageAsync = Promise.promisify(loadImage);If you aren't using Bluebird, you can use es6-denodeify for this.
Each Promise instance has a then() method on its prototype. This allows us to handle the result of the async task.
loadImageAsync('one.png')
.then(function(image) {
console.log('Image loaded', image);
}, function(err) {
console.error('Error loading image', err);
});then takes two functions, either of which can be null or undefined. The resolved callback is called when the promise succeeds, and it is passed the resolved value (in this case image). The rejected callback is called when the promise fails, and it is passed the Error object we created earlier.
Promises also have a .catch(func) to handle errors, which is the same as .then(null, func) but provides clearer intent.
loadImageAsync('one.png')
.catch(function(err) {
console.error('Could not load image', err);
});The .then() method always returns a Promise, which means it can be chained. The above could be re-written like so. If a promise is rejected, the next catch() or then(null, rejected) will be called.
In the following example, if the loadImageAsync method is rejected, the only output to the console will be the error message.
loadImageAsync('one.png')
.then(function(image) {
console.log('Image loaded', image);
return { width: image.width, height: image.height };
})
.then(function(size) {
console.log('Image size:', size);
})
.catch(function(err) {
console.error('Error in promise chain', err);
});In general, you should be wary of long promise chains. They can be difficult to maintain and it would be better to split the tasks into smaller, named functions.
Your then() and catch() callbacks can return a value to pass it along to the next method in the chain. For example, here we resolve errors to a default image:
loadImageAsync('one.png')
.catch(function(err) {
console.warn(err.message);
return notFoundImage;
})
.then(function(image) {
console.log('Resolved image', image);
});The above code will try to load 'one.png', but will fall back to using notFoundImage if the load failed.
The cool thing is, you can return a Promise instance, and it will be resolved before the next .then() is triggered. The value resolved by that promise will also get passed to the next .then().
loadImageAsync('one.png')
.catch(function(err) {
console.warn(err.message);
return loadImageAsync('not-found.png');
})
.then(function(image) {
console.log('Resolved image', image);
})
.catch(function(err) {
console.error('Could not load any images', err);
});The above tries to load 'one.png', but if that fails it will then load 'not-found.png'.
Let's go back to our original task of loading multiple images.
The Promise.all() method accepts an array of values or promises and returns a new Promise that is only resolved once all the promises are resolved. Here we map each URL to a new Promise using loadImageAsync, and then pass those promises to all().
var urls = ['one.png', 'two.png', 'three.png'];
var promises = urls.map(loadImageAsync);
Promise.all(promises)
.then(function(images) {
console.log('All images loaded', images);
})
.catch(function(err) {
console.error(err);
});Finally, things are starting to look a bit cleaner.
You may still be wondering where promises improve on the async approach. The real benefits come from composing promises across your application.
We can "pass the buck" by making named functions that return promises, and let errors bubble upstream. The above code would look like this:
function loadImages(urls) {
var promises = urls.map(loadImageAsync);
return Promise.all(promises);
}A more complex example might look like this:
function getUserImages(user) {
return loadUserData(user)
.then(function(userData) {
return loadImages(userData.imageUrls);
});
}
function showUserImages(user) {
return getUserImages(user)
.then(renderGallery)
.catch(renderEmptyGallery);
}
showUserImages('mattdesl')
.catch(function(err) {
showError(err);
});If you throw inside your promise chain, the error will be impliticly caught by the underlying Promise implementation and treated as a call to reject(err).
In the following example, if the user has not activated their account, the promise will be rejected and the showError method will be called.
loadUser()
.then(function(user) {
if (!user.activated) {
throw new Error('user has not activated their account');
}
return showUserGallery(user);
})
.catch(function(err) {
showError(err.message);
});This part of the specification is often viewed as a pitfall of promises. It conflates the semantics of error handling by combining syntax errors, programmer error (e.g. invalid parameters), and connection errors into the same logic.
It leads to frustrations during browser development: you might lose debugger capabilities, stack traces, and source map details.
For many developers, this is enough reason to eschew promises in favour of error-first callbacks and abstractions like async.
We can use .then() on a promise even after the asynchronous task is long complete. For example, instead of always requesting the same 'not-found.png' image, we can cache the result of the first request and just resolve to the same Image object.
var notFound;
function getNotFoundImage() {
if (notFound) {
return notFound;
}
notFound = loadImageAsync('not-found.png');
return notFound;
}This is more useful for server requests, since the browser already has a caching layer in place for image loading.
The Promise class also provides a resolve and reject method. When called, these will return a new promise that resolves or rejects to the (optional) value given to them.
For example:
var thumbnail = Promise.resolve(defaultThumbnail);
//query the DB
if (userLoggedIn) {
thumbnail = loadUserThumbnail();
}
//add the image to the DOM when it's ready
thumbnail.then(function(image) {
document.body.appendChild(image);
});Here loadUserThumbnail returns a Promise that resolves to an image. With Promise.resolve we can treat thumbnail the same even if it doesn't involve a database query.
Functions that return promises should always return promises, so the user does not need to wrap them in a try/catch block.
Instead of throwing errors on invalid user arguments, you should return a promise that rejects with an error. Promise.reject() can be convenient here.
For example, using our earlier loadImageAsync:
function loadImageAsync(url) {
if (typeof url !== 'string') {
return Promise.reject(new TypeError('must specify a string'));
}
return new Promise(function (resolve, reject) {
/* async code */
});
}Alternatively, you could use throw inside the promise function:
function loadImageAsync(url) {
return new Promise(function (resolve, reject) {
if (typeof url !== 'string') {
throw new TypeError('must specify a string');
}
/* async code */
});
}See here for details.
Although this guide uses bluebird, it should work in any standard Promise implementation. For example, using Babel.
Some other implementations:
For example, in Node/browserify:
// use native promise if it exists
// otherwise fall back to polyfill
var Promise = global.Promise || require('es6-promise').Promise;In addition to the the issues mentioned in throw and implicit catch, there are some other problems to keep in mind when choosing promises. Some developers choose not to use promises for these reasons.
One situation where promises are not yet a good fit is in small, self-contained npm modules.
- Depending on
bluebirdores6-promiseis a form of vendor lock-in. It can be a problem for frontend developers, where bundle size is a constraint. - Expecting the native
Promise(ES2015) constructor is also a problem, since it creates a peer dependency on these polyfills. - Mixing different promise implementations across modules may lead to subtle bugs and debugging irks.
Until native Promise support is widespread, it is often easier to use Node-style callbacks and independent async modules for control flow and smaller bundle size.
Consumers can then "promisify" your API with their favourite implementation. For example, using the xhr module in Bluebird might look like this:
var Promise = require('bluebird')
var xhrAsync = Promise.promisify(require('xhr'))Promises can introduce a lot of complexity and mental overhead into a codebase (evident by the need for this guide). In real-world projects, developers will often work with promise-based code without fully understanding how promises work.
See Nolan Lawson's "We Have a Problem With Promises" for an example of this.
Another frustration is that promises tend to work best once everything in your codebase is using them. In practice, you might find yourself refactoring and "promisifying" a lot of code before you can reap the benefits of promises. It also means that new code must be written with promises in mind — you are now stuck with them!
For a comprehensive list of Promise resources and small modules to avoid library lock-in, check out Awesome Promises.
- JavaScript Promises: There and back again
- We Have a Problem With Promises
- You're Missing the Point of Promises
MIT, see LICENSE.md for details.