ESLint plugin with ^? Twoslash, $ExpectError, and $ExpectType type assertions. ✨

9001;
// ^? number

// $ExpectError
const value: string = 9001;

// $ExpectType number
9001;

Make sure you have TypeScript and @typescript-eslint/parser installed, then install the plugin:

npm i -D eslint-plugin-expect-type

See typescript-eslint's Getting Started docs for how to run ESLint on TypeScript files.

Add the following options to your ESLint configuration file:

{
  "extends": ["plugin:expect-type/recommended"],
  "plugins": ["eslint-plugin-expect-type"]
}

Then, you'll be able to use ^?, $ExpectError, $ExpectType, and $ExpectTypeSnapshot comments in code assert on types.

💼 Configurations enabled in.
✅ Set in the recommended configuration.
🔧 Automatically fixable by the --fix CLI option.
💭 Requires type information.

You might consider using other popular type assertion libraries in the TypeScript ecosystem:

• expect-type: Provides functions that return assorted generic type assertion methods, such as expectTypeOf('abc').toMatchTypeOf<string>().
• ts-expect: Provides generic type assertion function, used like expectType<string>('abc')().
• tsd: Allows writing tests specifically for .d.ts definition files.

Many thanks to @ibezkrovnyi for creating the initial version and core infrastructure of this package! 💖