[no-undef] Exported interface is marked as unused

[timdp]

What code were you trying to parse?

https://github.com/timdp/standard-typescript-interfaces

What did you expect to happen?

I'm trying to use Standard with TypeScript.

The compiler allows me to export an interface from types.ts and import it into index.ts, but the linter and formatter complain.

My goal is actually to create a shared types package in a monorepo, but I've simplified it to a plain old module.

I'm well aware that interfaces don't exist in JavaScript, but given that tsc allows me to do this, ESLint should understand it as well, right?

What actually happened?

$ standard '*.ts'
standard: Use JavaScript Standard Style (https://standardjs.com)
  .../index.ts:1:10: 'Foo' is defined but never used.
  .../types.ts:11:10: 'Foo' is not defined.

Versions

package	version
@typescript-eslint/parser	1.1.0
TypeScript	3.2.4
ESLint	5.4.0
node	10.15.0
npm yarn	1.13.0

[j-f1]

Make sure you’ve enabled the @typescript-eslint/no-unused-vars rule from the @typescript-eslint plugin.

[timdp]

That would avoid the unused-import warning but the type is actually used. Note the second error.

[j-f1]

Does installing the latest @typescript-eslint/eslint-plugin help?

[timdp]

I just updated the code to do that. Same error though.

[bradzacher]

Could you please chuck a succinct repl case into the OP?

It's a bit hard for us to reproduce and fix the issue if we have to checkout your entire repo to investigate.

[armano2]

^

https://github.com/timdp/standard-typescript-interfaces

[bradzacher]

I'm assume standard just uses the recommended config?
This is what I mean by creating a succinct repl - the level of indirection with a tool makes it a bit harder to assess. Have to switch between a repo and its files.. not having rule names in the output..

[bradzacher]

Reviewing the source for no-undef, it looks to me like a bug with the scope extensions that we're doing - the interface is not added to the global variable scope.
https://github.com/eslint/eslint/blob/master/lib/rules/no-undef.js#L59-L73

[timdp]

Cool, cool. Still need the REPL demo then?

[armano2]

@bradzacher to be precise, non of "type" nodes are not added to scope,
type Foo, interface, typeParameters, typeArguments and so on.

[mubaidr]

Any update?

[bradzacher]

I would suggest not using no-undef on typescript code.
The typescript compiler should catch all of the cases itself, so you shouldn't need the rule.

We'll look into a fix, though it will not be soon.
Scope analysis is a difficult thing when you add in the multiple and shared scopes that typescript adds.

[mubaidr]

^ Yes, makes sense. 👍

[meikidd]

Same issue here. Below is my code, it is allowed by typescript, but the linter deny it.

type User = {
  id: number;
  name: string;
};

export default User;

linter error:

./src/models/user.ts
  Line 6:  'User' is not defined  no-undef

[mockdeep]

We're running into this same issue. And, unfortunately, TypeScript does not catch the undefined globals because in this case we're dealing with JQuery. In the type defs for JQuery, it is declared as a global, so TypeScript ignores references to it. For various reasons, we're trying to move to locally importing JQuery in our files, so the no-undef rule is pretty valuable to making sure we don't miss anything.

The workaround for the moment, was to move the type it was complaining about to being a global type def in a .d.ts file.

[bradzacher]

Merging into #1856