Polyfill as needed with polyfill-service
August 06, 2018
How to maximize the performance of loading polyfills for your application users.
So what did I do to fix my IE10 bug? Well, one thing that really bugs me is that I have to ship all this code for polyfills to all browsers even if they do support these features. But a few years ago I heard of > a service that was able to ship polyfills that are relevant only to the browser requesting them. I created my own endpoint that uses the module > that powers that service and I'll write about that next week!
That's today's newsletter! I'll explain how I created a
that users need and no more.
With the way I have my usage of polyfill-service configured today, if I make a
polyfill.js using Internet Explorer 10 (the lowest version of IE
that we support), the response is 60.2kb! If you're unfamiliar with the impact
this can make, I suggest you read
by Addy Osmani (or
watch a talk version here). To
put this in terms you may appreciate, this will take users in emerging markets
about a full second just to download, then you have to take the content they've
downloaded and parse/compile/run it which can take even longer especially for
individuals using lower-end phones.
The state of the art with polyfills is to include those polyfills in your
bundle.jsfile (in fact, lots of apps are just using all of
core-js which is
84.2 kb of minified JS). This
regardless of what browser they're using. But let's take a look at
browser usage statistics. Your stats may vary
depending on your users, but if your app is typical of the global averages, then
you have maybe 5% of your users who need more than a handful of kbs worth of
polyfills. Most of your users will be using modern, evergreen browsers that
support most of the features you're using. So you're making users who are
running modern browsers pay a "tax" for your site supporting those 5% of users
who wont/can't upgrade.
If I run Chrome 67 on my
polyfill.js file, it comes back
basically empty. By using
polyfill-service, only the browsers which need the polyfills receive them.
This means that they can use my app quicker and I'm not taking up some of your
bandwidth to download stuff you don't need (which actually means saving people
actual dollars if they don't have unlimited data).
Another aspect of using something like polyfill-service is because my polyfills
live in a completely different file from my
bundle.js, I can have it cached
forever, so users only need to download it once and never need to download it
again. So even for users on bad networks, they'll benefit from not having to
expend resources re-downloading a file that will never change.
The polyfill.io service from Financial Times is awesome, but with no SLA (service level agreement), many companies can't rely on it for mission-critical applications. Luckily, the module that powers it is completely open source so you can set up your own service in-house in a pretty straightforward way and that's exactly what I did.
With the app I'm working on right now (paypal.me), we have
a server that's responsible for some light server-rendering for SEO purposes.
Basically, our server is a NodeJS server using KrakenJS
(a wrapper on top of express), so I added a
gethandler to the express app:
And with the
getBrowserPolyfill is a typical express route handler:
There's a little bit more to it, but this is the basic idea. So let's talk about a few aspects of this solution.
So the polyfill-service module needs to know what the user agent string is to
determine what the
include). So I pass
req.headers['user-agent'] as that value, though I allow
ua query string to override this and I have a fallback to IE 9 just in
case. And in the case that polyfill-service encounters a user agent it doesn't
recognize, I have it configured to just treat it as if it needs all the
polyfills (via the
unknown: 'polyfill' option).
There are a LOT of features that polyfill-service supports out of the box. It
defaults to the most useful ones, but it's a good idea to configure it. At first
I thought: "Hey, let's just have it support everything." But then I found out
that if you asked it to polyfill everything it could, it'll get HUGE (mostly
because it actually supports
Intl with every language pack which is kinda
reeeally big). So I ended up with specifying
default-3.6 as the features config. That's working great and
supports everything that I care to support.
This one's a bit interesting. So that
shouldCacheAggressively is a bit
dangerous, so here's what I do... Because we're server-rendering the page, I can
actually generate the URL for the polyfill. It ends up looking like this (for IE
There are two query strings on there:
v which is associated to a
that I have hard-coded. This allows me to break the cache in the event of an
emergency if we need to change the config or something.
I also generate it with the
ua which is the user agent as part of the query
string for the
polyfill.js file. Remember how I mentioned earlier that I allow
uqquery string to override
req.headers['user-agent']? So that's what
this is doing. The reason I do this is for caching. With such a specific URL, I
can safely cache this forever. If the user upgrades (or downgrades!?) their
browser, but the cache isn't deleted, then this URL is changed and the old
cached version isn't used.
One "fun" experience I had while building this involved polyfill-service not playing nice with the way that babel transpiles classes. Follow that twitter thread and github issues linked for a "fun" time of your own... 😅
Best of luck to you!
- More than you want to know about ES6 Modules @ Learn to Code Websites and Apps Meetup (remote)
- ES6 and Beyond Workshop Part 1 at PayPal (Jan 2017)
- ES6 and Beyond Workshop Part 2 at PayPal (March 2017)
- Code Transformation and Linting
- Writing custom Babel and ESLint plugins with ASTs
Things to not miss:
express-async-errors- a nice package that allows me to use
async/awaiton express route handlers/middleware without worrying about rejected promises being ignored and making my server hang :)
- ReactJS Denver: Confidently Testing React Apps — A great talk at React Denver by Matt Parrish.
- The Surface Book 2 is everything the MacBook Pro should be by Owen Williams — I'm getting more and more convinced that my next computer is NOT going to be a MacBook. Here's hoping that the Surface Book can get me 32 GB of RAM before I need another computer!