Using eslint alongside the Firefox Hello code base to help productivity

On Firefox Hello, we recently added the eslint linter to be run against the Hello code base. We started of with a minimal set of rules, just enough to get us something running. Now we’re working on enabling more rules.

Since we enabled it, I feel like I’m able to iterate faster on patches. For example, if just as I finish typing I see something like:

eslint syntax error in sublime I know almost immediately that I’ve forgotten a closing bracket and I don’t have to run anything to find out – less run-edit-run cycles.

Now I think about it, I’m realising it has also helped reduced the amount of review nits on my patches – due to trivial formatting mistakes being caught automatically, e.g. trailing white-space or missing semi-colons.

Talking about reviews, as we’re running eslint on the Hello code, we just have to apply the patch, and run our tests, and we automatically get eslint output:

eslint output - no trailing spacesHopefully our patch authors will be running eslint before uploading the patch anyway, but this is an additional test, and a few less things that we need to look at during review which helps speed up that cycle as well.

I’ve also put together a global config file for eslint (see below), that I use for outside of the Hello code, on the rest of the Firefox code base (and other projects). This is enough, that, when using it in my editor it gives me a reasonable amount of information about bad syntax, without complaining about everything.

I would definitely recommend giving it a try. My patches feel faster overall, and my test runs are for testing, not stupid-mistake catching!

Want more specific details about the setup and advantages? Read on…

My Setup

For my setup, I’ve recently switched to using Sublime. I used to use Aquamacs (an emacs variant), but when eslint came along, the UI for real-time linting within emacs didn’t really seem great.

I use sublime with the SublimeLinter and SublimeLinter-contrib-eslint packages. I’m told other editors have eslint integration as well, but I’ve not looked at any of them.

You need to have eslint installed globally, or at least in your path, other than that, just follow the installation instructions given on the SublimeLinter page.

One configuration I change I did have to make to the global configuration:

  • Open up a normal javascript (*.js) file.
  • Select “Preferences” -> “Settings – More” -> “Syntax Specific – User”
  • In the file that appears, set the configuration up as follows (or whatever suits you):
{
  "extensions":
  [
    "jsm",
    "jsx",
    "sjs"
  ]
}

This makes sure sublime treats the .jsm and .jsx files as javascript files, which amongst other things turns on eslint for those files.

Global Configuration

I’ve uploaded my global configuration to a gist, if it changes I’ll update it there. It isn’t intended to catch everything – there’s too many inconsistencies across the code base for that to be sensible at the moment. However, it does at least allow general syntax issues to be highlighted for most files – which is obviously useful in itself.

I haven’t yet tried running it across the whole code base via eslint on the command line – there seems to be some sort of configuration issue that is messing it up and I’ve not tracked it down yet.

Firefox Hello’s Configuration

The configuration files for Hello can be found in the mozilla-central source. There’s a few of these because we have both content and chrome code, and some of the content code is shared with a website that can be viewed by most browsers, and hence isn’t currently able to use all the es6 features, whereas the chrome code can. This is another thing that eslint is good for enforcing.

Our eslint configuration is evolving at the moment, as we enable more rules, which we’re tracking in this bug.

Any Questions?

Feel free to ask any questions about eslint or the setup in the comments, or come and visit us in #loop on irc.mozilla.org (IRC info here).