[no-undef] Linter does not report error when using undefined variables.

[gkjohnson]

Repro

package.json

{
  "name": "typescript-test",
  "scripts": {
    "lint": "eslint ./src/* --ext js --ext ts"
  },
  "devDependencies": {
    "@typescript-eslint/eslint-plugin": "^1.11.0",
    "@typescript-eslint/parser": "^1.11.0",
    "eslint": "^6.0.1",
    "typescript": "^3.5.2"
  },
  "eslintConfig": {
    "parser": "@typescript-eslint/parser",
    "plugins": [
      "@typescript-eslint"
    ],
    "rules": {
      "no-undef": 1
    }
  }
}

src/test.d.ts OR src/test.ts

export class TestClass {
    field: UndefinedClass;
}

Expected Result

A no-undef rule error is reported because UndefinedClass is being used but has not been defined.

Actual Result

No errors are reported.

Additional Info

The error is correctly reported when doing the equivalent in a js file.

Versions

package	version
@typescript-eslint/eslint-plugin	1.11.0
@typescript-eslint/parser	1.11.0
TypeScript	3.5.2
ESLint	6.0.1
node	12.4.0
npm	6.9.0

[bradzacher]

We suggest turning off no-undef for typescript projects.
It can only ever report errors that are handled automatically (and better) by the typescript compiler itself.

[gkjohnson]

It can only ever report errors that are handled automatically (and better) by the typescript compiler itself.

I'm not sure I follow -- by "it" do you mean the typescript eslint plugin? To elaborate I expect that this feature would be implemented as a new rule (such as @typescript-eslint/no-undef) that reports the TS2304 error the way the compiler / VSCode does for files.

Thanks!

[bradzacher]

Our parser is only a parser. It leverages the typescript compiler to parse and generate an AST, and provide type information. However we don't currently have have access to the diagnostics typescript provides, nor can we access things like its scope analyser.

As it stands right now the rule maintains its own logic for analysing scope, which has no idea about typescript type scope.

If you're looking for a way to have typescript compiler error TS2304 reported, the best way is via running the typescript compiler..

[kaicataldo]

For what it’s worth, we turned no-undef off and just let tsc report this at my last job and it was never an issue. Most popular editors are able to report errors reported by tsc, and we would run tsc —noEmit and ESLint during the linting step of our CI process.

[bradzacher]

That was the implication of the first comment.
Recommend just turning this rule off because the typescript compiler can report the problem better than a custom written rule ever could.

Considering this rule's functionality is completely covered by the typescript compiler, rewriting the rule to work properly is low on the list of priorities...

[gkjohnson]

Got it -- thanks for the clear explanation! I'll look into using the typescript compiler to catch these types of issues.

[bradzacher]

Merging into #1856