False positives + query ideas for variables with leading underscores.

[s0]

js/unused-local-variable has some false positives for variables that are intentionally unused, e.g. for array destructuring, etc...

An example can be seen in Shopify/quilt#434

It's a convention that local variables with leading underscores are designed to be unread, e.g:

const [a, _, c, d, e, f] = something
// or
const [a, _b, _c, d, e, f] = something

Moreover, tslint recently temporarily deprecated their no-unused-variable rule in favour of the new compiler flags which take leading underscores into account.

I suggest we:

• ignore variables that start with an underscore in js/unused-local-variable
• create a new query that finds variables that suggests they should be unused, but are actually used.

[xiemaisi]

It's a convention (...)

I'm sure you're right, but could we have a reference for that convention? I'd also like to point out that in the examples you give you should be able to just omit the _* variables altogether:

const [a, , c, d, e, f] = something

[s0]

I'm sure you're right, but could we have a reference for that convention?

Here are a few of places:

• Typescript compiler's behaviour for compiler flags --noUnusedParameters and --noUnusedLocals

  Parameters declaration with names starting with _ are exempt from the unused parameter checking. e.g.:

  function returnNull(_a) { // OK
      return null;
  }

  (If we decide it's not a widely-enough used convention for JS, we could potentially enable it for TS only)

• ESLint documentation on no-unused-vars. Uses prefix underscores as an example for the argsIgnorePattern option:

  The argsIgnorePattern option specifies exceptions not to check for usage: arguments whose names match a regexp pattern. For example, variables whose names begin with an underscore.

• Ruby's community coding conventions:

  Prefix with _ unused block parameters and local variables. It's also acceptable to use just _ (although it's a bit less descriptive). This convention is recognized by the Ruby interpreter and tools like RuboCop and will suppress their unused variable warnings.

I'd also like to point out that in the examples you give you should be able to just omit the _* variables altogether

Ah yes!

[xiemaisi]

OK, thanks, that looks fairly widely accepted.

If we decide it's not a widely-enough used convention for JS, we could potentially enable it for TS only

We could, but to be honest I doubt we'll lose any interesting results by enabling it for JS as well.