Thou shalt polyfill
Recently, I had a look at a JSConf 2014 talk by Sebastian Markbage (engineer at Facebook and TC39 member) on "minimal surface APIs". What is that? Why does it require an article?
Use polyfills instead of limiting yourself with tools that embed the required set of backward-compatible machinery.
What is a minimal surface API?
First off, API here is not to be understood in the REST sense of the term. We're talking software API, like the number of methods and mechanisms you need to know to achieve a particular goal.
A minimal surface API requires you to forget about the differences of
implementation. But it should not be taken for abstraction, because too often,
abstraction itself adds entropy to APIs. Sebastian gives the perfect example of
Underscore, that makes you use
Array.prototype.map already exist.
This is too complicated a solution. According to Sebastian Markbage (which I
humbly second here), a better solution would have been to monkey patch
Array.prototype, despite what we've heard and/or read in the past about
monkey patching native prototypes in JS.
Why is it important?
Though we love the "Move the Web forward" trend, that tends to explicitly display to outdated browsers' users a message stating that they could "browse happy" (understood as "you'd better change your computer"), a lot of companies have to deal with hundreds if not thousands of (sometimes) very old computers, alongside very heavy security measures that prevent them form being able to update to the latest version of Chrome/Firefox (what else?).
We're not discussing their choices here, but we're facing a fact: You can't force your users to use your favorite browser. Period.
But it would be a shame to limit yourself to outdated browsers and prevent the more "technologically advanced" of your users to enjoy the niceties they embed, right?
You don't have to assume that all your users are using sh**ty browsers, just make it so they all get as many decent APIs as possible. That will make for a better experience for the users of modern browsers and it will also ease the transition when you won't need to use a particular polyfill anymore.
In a word, polyfills favor graceful degradation to progressive enhancement.
How do I...
Know if feature X is cross-browser?
Go to caniuse.com. It's a complete matrix of browsers and features.
You can and should also refer to the MDN which provide compatibility matrices for JS and CSS as well as atomic polyfills when they exist.
Know if such a feature is available at runtime?
Use Modernizr, it will let you load your polyfills only if needed. Though most polyfills will already do this part of the job, it allows to have a clear in-app context of what's possible and eventually switch solutions based on what's been detected (eg. Why load a touch API on a desktop?). Indispensable.
Find the correct polyfill?
Modernizr offers a dedicated wiki page with an impressive number of polyfills for most of the features it covers. In the most extreme cases, Google "xyz polyfill", go to github and you'll certainly find what you're looking for.
A minimal surface API is an API where there are no two single functions that do the same thing. Take care of it for your sanity, for the clarity of your code and for your project to be kept updated, because polyfills are meant to be removed after some time, after all, right?
This approach has been in the running at Facebook for quite some time; for instance, React is built with the EcmaScript 6 syntax. There are other projects that follow this track, as Ember, which we're using greatly here.
If you're in line with the contents of this article and want to have a chat with us, feel free to ping us, we're always available... and hiring :)