Check TypeScript Definitions

Unit tests for .d.ts TypeScript definitions in your JavaScript open source library.

It is useful for non-TypeScript project, which want to provide good typing support for TypeScript users and good autocompletion for IDE and text editors.

It became especially useful if you have complex types with generics, like we have in Nano Events or Storeon.

// Negative test: test/index.errors.ts
import lib = require('../')

interface Events {
  'set': (a: string, b: number) => void
}
// THROWS Expected 3 arguments, but got 2
lib.on<Events>('set', 2)

// Positive test: test/index.types.ts
import lib = require('../')

interface Events {
  'set': (a: string, b: number) => void
}
lib.on<Events>('set', 'prop', 1)

Usage

1. Add .d.ts files with TypeScript definitions for your JS library. You can check example in Nano Events.

2. Install check-dts:

   npm install --save-dev check-dts

3. Create test/index.types.ts for positive tests and write correct TypeScript. You can test IDE autocompletion in that file.

4. Run npx check-dts to test new file.

5. Create test/index.errors.ts for negative tests and write wrong types there. See the next section for details.

6. Run npx check-dts to test both files.

7. Add check-dts to npm test to test types on CI:

     "scripts": {
   -   "test": "jest && eslint ."
   +   "test": "jest && eslint . && check-dts"
     }

8. If your library requires additional TypeScript option, you can define them for tests in tsconfig.json.

Writing Negative Test

Put a comments like // THROWS some error messages above the line, where do you expect an error from TypeScript.

Write code, where do you expect TypeScript to tell you about the error.

import lib = require('../')

interface Events {
  set: (a: string, b: number) => void
}
lib.on<Events>('set', 2)

In this case, we expect error message Expected 3 arguments, but got 2. So we should add comments. You can put only part of the error message to the // THROWS comment.

  import lib = require('../')

  interface Events {
    set: (a: string, b: number) => void
  }
+ // THROWS Expected 3 arguments, but got 2
  lib.on<Events>('set', 2)

If TypeScript will not find the error or will throw a different error, check-dts will fall with a description:

$ npx check-dts
✖ Check types
✖ test/index.errors.ts

  errors.ts:7:23: Wrong error
  Expected: Expected 0 arguments, but got 1
  Got: Expected 3 arguments, but got 2.