Cleanup: add no-unused-vars and imports eslint rules and fix the codebase

[yannikmesserli]

No issue.

Description

This PR adds the no-unused-vars and imports eslint rules. Then, I ran eslint --fix and finally fixed the remaining issue that couldn't be automatically fixed

Changes

• Added a new npm package unused-imports which is better than the default rules
• Enabled the rules in .eslintrc.js with ["error", { "ignoreRestSiblings": true }]
• Ran eslint --fix
• Fixed the remaining issues

Screenshots

n/a

Unit tests

They should still all pass

Functional tests

It's probably worth testing quickly the entire app, to see that nothing have been broken.

[yandzee]

Feels like its not an issue of inheritance, more like an issue of linter. Would that work if you replaced _isFirst with _ ?

[yannikmesserli]

We can _ but then the next person landing on this code will ask himself why there is an unused variable, will likely remove it, and broke again the inheritance. So we should keep the comment to explain why there is an unused variable here until we fix the inheritance, a.k.a we don't add unused variable because parents might use them.

[yandzee]

I agree with most of this changes, but I don't think that function signatures should be changed. I suggest to try to find an options for linter to disable such behavior.

Its ok to strip unused variables like this:

const fn = () => {
  const thisIsUnused = 'random string';

  return 42;
}

but its not ok to strip function arguments because by this changes you have risks even to break an API (case of messageBuilder args)

[yandzee]

I don't see much advantages on such changes. Feels like there should be a separate rule (or options for existing rules) which could change this point: its wrong to remove unused function args because obviously they can be a part of function signatures.

[yannikmesserli]

but I don't think that function signatures should be changed.

Why not? I really don't see a case where an unused parameter of function could be useful. Maybe you can provide an example?

The problem with removing this lint rule is that it's really common for developers to add parameters when they start a new function and not using some of them at the end, after a few massage, while not noticing they're left unused. Think of, for example, a forEach loop: you would think first you'll need the index and ends up without using it. As the code base grows, this will clutter the code, making harder to read it, and therefore, harder to spot bugs.

I suggest that if we find a clear case of a needed unused parameter, we then add a lint exception comment WITH a clear comment to explain why this parameter is absolutely needed, so that no other developer will remove in the future after you.