Detect Unused Variables

This recipe explains how to configure Wotan to detect unused variables and imports.

The Problem

TypeScript has the option noUnusedLocal to detect unused variables and imports. The resulting findings are treated like any other compile error. Sometimes you need to keep a variable or import. For example if you are compiling with the declaration option enabled and get the error XXX has or is using name 'Foo' from external module "../bar" but cannot be named. As a workaround you could just drop in some // @ts-ignore comments. But that also disables all other errors on that line.

TSLint's rule no-unused-variable uses TypeScripts noUnusedLocals internally and reports the result as lint errors. The rule tries to work around Microsoft/TypeScript#9944 but isn't very effective as there are still false positives. In addition that rule has some pretty nasty side effects palantir/tslint#2736 palantir/tslint#2649 and is pretty damn slow.

The Solution

Wotan has a builtin rule to report type checking errors as lint findings: typecheck. You can use this rule together with a special tsconfig.json to report noUnusedLocals errors as lint findings.

1. Enable typecheck in your .wotanrc.yaml:

   rules:
     # your other rules go here
     typecheck: error # could also be 'warning'
   

2. Disable noUnusedLocals in your tsconfig.json you use to compile your program.
3. Add a new tsconfig.lint.json (you can choose another name if you prefer) that extends your tsconfig.json (the one used for compiling) and enable noUnusedLocals

   {
     "extends": "./tsconfig.json", // the name of your existing tsconfig.json
     "compilerOptions": {
       "noUnusedLocals": true
     }
   }
   

4. Run the linter with your new tsconfig.lint.json

   wotan -p tsconfig.lint.json
   

You will now see 'Foo' is declared but its value is never read. as lint findings instead of compile errors. You can now use a // wotan-disable-line typecheck comment to suppress the finding in that line.

You continue to use the original tsconfig.json for compiling. That way TypeScript can report any other error while unused variables or imports are reported by Wotan.