Update eslint to latest versions

[KaliedaRik]

What does this change?

This PR updates the ESLint tool to the latest versions.

Note that this includes moving to ESLint's new config system - see relevant posts about this made to the ESLint blog .

The primary aim of this update is to get rid of some Dependabot security alerts, and to lower technical debt going forward.

Future Health work

• This PR works because it is turning off (potentially essential) linting checks - eg: "no-fallthrough": "off"
• The current output shows where we are using eslint-disable directives in files. In an ideal world, we should not need those directives.
• Future health work should attempt to address these issues by moving each check from off to warn and then fixing the code that is causing the warn message.
• Once those warnings are fixed, we can move the check (if essential) from warn to error to prevent anyone reintroducing such an error into the code base.

How to test

• Run yarn lint locally - there should be plenty of warnings, but no errors reported.
• Run the PR in the CODE environment to see if the new system brakes any CI steps.
• Test in CODE - everything should operate as before

[rebecca-thompson]

Looks good. Is it worth migrating the custom config to use @guardian/eslint-config / @guardian/tsconfig at some point?

[KaliedaRik]

There's an issue with the CI build, which I can replicate locally:

• Checkout the branch
• Run yarn to install dependencies
• Run yarn lint - should pass (24 warnings - all of which are Unused eslint-disable directive)
• Run yarn build - this step fails

The reported CI failure occurs when attempting to build the client module and fails specifically because of a type-check error:

... which I can replicate when building locally:

[KaliedaRik]

Interesting aside following on from above comment. If I run yarn lint after running yarn build the lint tests report a whole bunch of fresh errors - all in the newly created dist folders:

This doesn't affect CI because the lint step runs before the build step, but it does make me wonder what the bundler is doing to our source code.