Switch from JSHint to ESLint

[pdehaan]

See mozilla/fxa-content-server#2550 and mozilla/fxa-content-server#2537 (comment)

[TDA]

@pdehaan I would be happy to do this once I am done with 2550. I am assuming the same steps? All I would need to do is clone this locally and make similar changes and push and make a PR?

[pdehaan]

@TDA: I think so. Probably want to verify w/ @shane-tomlinson @zaach and @vladikoff to see if they want to keep both JSHint AND ESLint for a brief time, or if we just use ESLint and remove the old linter all at once.

[shane-tomlinson]

@TDA: I think so. Probably want to verify w/ @shane-tomlinson @zaach and @vladikoff to see if they want to keep both JSHint AND ESLint for a brief time, or if we just use ESLint and remove the old linter all at once.

Let's keep the two running concurrently for maybe a week, then rip out JSHint.

[pdehaan]

Also, this was the approach I was thinking of before w/ using an ESLint shareable config:
https://github.com/pdehaan/eslint-config-fxa

Basically we'd have one module of rules that we can share between projects and extend.

Then our .eslintrc file would be something like this:

extends: fxa

Or for you JSON-thusiasts:

{
  "extends": "fxa"
}

With possibly overriding a few rules, envs as needed.

[vladikoff]

Basically we'd have one module of rules that we can share between projects and extend.

Yes please!

[zaach]

Cool idea @pdehaan.

[pdehaan]

OK, I was testing this locally in a couple fxa-* repos last night and tweaking my client/server rules and have a proof of concept PR at mozilla/fxa-auth-db-server#148

extends: fxa/server

rules:
  handle-callback-err: 0
  semi: 0

It uses all the default "fxa" and "fxa/server" rules in eslint-config-fxa and then disables two additional rules, handle-callback-err and semi. And if we clean up ~10 semicolons we could reenable the semi rule and have it enforce/reject semicolon usage. So each repo has a good set of "default rules" and still has the flexibility to use custom rules as needed. Plus because child .eslintrc files inherit rules from their parents, it's easy enough to add .eslintrc files in subdirectories to add custom environments for /tests/ or whatever.

... And the coolest part, is that I basically was able to copy all of @TDA's rules from the the fxa-content-server repo's .eslintrc file and this "just worked"!