Testing Typescript Types

Lost confidence in types you created? You came to the right place.

What

A library for testing very complex types: with generics, unions, intersections, conditionals, inferings etc.

Why

You don't need to test if primitive types like string or boolean work correctly. Because they were tested by the TypeScript team. Now when you create ultra-complex type you are not that sure about type working as intended anymore.

Who

For those typescript developers who want to be sure that the type they created works as expected.

Example

Below is the structure of test files.

test
|__qur-typetest
   |__file1.ts
   |__file2.ts
   |__file3.ts

For simplicity we show example with simple types.

file1.ts:

const str: string = "foo"

file2.ts:

// tt:throws:Type '1' is not assignable to type 'string'.
const str: string = 1

file3.ts:

// tt:throws:Foo some wrong error message
const str: string = 1

Note: each test file should have at least one export or import expression. This way Typescript compiler would consider those files as modules, i.e. isolated code blocks. Otherwise for the example above tsc would generate errors Cannot redeclare block-scoped variable 'str'. Because of this reason, add this in the end of each files:

export {}

Result:

  ✓ test/qur-typetest/file1.ts
  ✓ test/qur-typetest/file2.ts
  ✗ test/qur-typetest/file3.ts

  test/qur-typetest/file3.ts was expected to fail with a message

    Foo some wrong error message

  but failed with

    Type '1' is not assignable to type 'string'.

  at test/qur-typetest/file3.ts:2:55

  1/3 failed
  2/3 passed

Install

npm install @qur/typetest --save-dev

Add test/qur-typetest to the exclude option of your tsconfig.json

"compilerOptions": {},
"exclude": [
  "test/qur-typetest"
]

Usage

Add a script to package.json

"scripts": {
  "typetest": "qur-typetest"
}

Do the following

npm run typetest -- init
echo "const str: string = 1" > test/qur-typetest/myTypeTest.ts
npm run typetest

CI

In order to include types testing to your Continuous Integration process, you should run typetest with other tests. Your test script in package.json could look like this:

"scripts": {
  "test": "mocha && typetest"
}

TODO

• Testing types
• tests of this very library
• prettier errors
• Cli command "init" to create test/qur-typetest folder
• Cli command "init" adds exclude to tsconfig.json
• Cli command "create" to create test files

FAQ

Why should I add at least one export/import to a test file?

Because otherwise typescript thinks your files are parts of one code block, not an isolated module. Presence of export/import make typescript think that the file is a module and thus, it is isolated. You can omit using export/import until you actually encounter the described problem.

Name

Why "qur-typetest" and not just "typetest"? Because "qur" is going to be an ecosystem of libraries, it is work in progress now. Also this is done in order to avoid collisions with probably existing "test/typetest" folders. Also, npm just not allowed to create "typetest" named library.