Check TypeScript Definitions

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

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

It becomes especially useful for 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 this file.

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

5. Create test/index.errors.ts for negative tests and add an incorrect usage of your library recording to TypeScript. 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 an additional TypeScript option, you can define it for tests in tsconfig.json.

Writing Negative Test

Add code where you expect TypeScript to report an error. Make sure to add a line above the expected error like // THROWS some error messages

import lib = require('../')

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

In this case, we expect the 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 does not report the error or reports a different error, check-dts will fall with a description:

$ npx check-dts
✖ Check types

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

CLI Options

Test all .ts and .js files in project and run *.types.ts and *.errors.ts tests:

$ npx check-dts

You can test only specific files by:

$ npx check-dts **/*.tsx?
$ npx check-dts **/*.ts !**/index.ts