Add "Standard" javascript linting

[fofr]

"Standard" was added to govuk_frontend_toolkit here:
alphagov/govuk_frontend_toolkit#334

By adding Standard to govuk-lint we can lint Javascript across our apps using the same config.

@NickColley @tvararu @dsingleton

[dsingleton]

Sounds reasonable to me.

[NickColley]

Is it possible to do this only for lines changed or similar? I imagine this'll make a lot of projects frustrating for people to update to start with.

[fofr]

How can we package up the npm module in this gem?

[fofr]

One approach would be to use eslint-rails with the standard JS conf:
https://github.com/appfolio/eslint-rails
https://github.com/feross/eslint-config-standard/blob/master/eslintrc.json

Some challenges I foresee with using eslint-rails:

• Can't easily customise the files being linted or those being excluded
• Can't alter location of config file
• The gem isn't built to be run from within a gem/custom CLI – it does a lot of Rails magic for determining which JS files to lint based on what's in application.js

[fofr]

A WIP to add Javascript linting based on the eslint-rails approach for getting eslint to run in a Ruby environment: https://github.com/alphagov/govuk-lint/compare/javascript-linting

I've stopped this POC because the standard linting config is not simply a list of JSON rules but also a set of plugins that must be included and run. It also feels brittle.

I'd prefer something that uses the npm standard package. I've raised an issue on govuk-puppet regarding the Node version available on Jenkins and VMs: alphagov/govuk-puppet#5942

[tvararu]

@fofr thanks for looking into this Paul!

Now that Rails 5.1 is released with support for webpack and yarn built in, would that be an easier route for this?

[fofr]

@tvararu Yes, I think that’s promising too. It looks like there are some webpack standard libs (eg https://github.com/timoxley/standard-loader).

I'm not sure how webpack/yarn dependencies can be packaged into a gem like govuk-lint.

We'll still need to get Node upgraded first. Requires Node.js 6.4.0+

[dankmitchell]

FYI - We now use XO for linting.

[theseanything]

See #51