# tsd-check [![Build Status](https://travis-ci.org/SamVerschueren/tsd-check.svg?branch=master)](https://travis-ci.org/SamVerschueren/tsd-check)

> Check TypeScript type definitions


## Install

```
$ npm install tsd-check
```


## Usage

Let's assume we wrote a `index.d.ts` type definition for our concat module.

```ts
declare const concat: {
	(value1: string, value2: string): string;
	(value1: number, value2: number): string;
};

export default concat;
```

In order to test this definition, add a `index.test-d.ts` file.

```ts
import concat from '.';

concat('foo', 'bar');
concat(1, 2);
```

Running `npx tsd-check` as a command will verify that the type definition works correctly.

Let's add some extra [assertions](#assertions). We can assert the return type of our function call to match a certain type.

```ts
import {expectType} from 'tsd-check';
import concat from '.';

expectType<string>(concat('foo', 'bar'));
expectType<string>(concat(1, 2));
```

The `tsd-check` command will succeed again.

We change our implementation and type definition to return a `number` when both inputs are of type `number`.

```ts
declare const concat: {
	(value1: string, value2: string): string;
	(value1: number, value2: number): number;
};

export default concat;
```

If we don't change the test file and we run the `tsd-check` command again, the test will fail.

<img src="screenshot.png" width="1330">

### Top-level `await`

If your method returns a `Promise`, you can use top-level `await` to resolve the value instead of wrapping it in an `async` [IIFE](https://developer.mozilla.org/en-US/docs/Glossary/IIFE).

```ts
import {expectType, expectError} from 'tsd-check';
import concat from '.';

expectType<Promise<string>>(concat('foo', 'bar'));

expectType<string>(await concat('foo', 'bar'));

expectError(await concat(true, false));
```

### Test directory

When you have spread your tests over multiple files, you can store all those files in a test directory called `test-d`. If you want to use another directory name, you can change it in `package.json`.

```json
{
	"name": "my-module",
	"tsd-check": {
		"directory": "my-test-dir"
	}
}
```

Now you can put all your test files in the `my-test-dir` directory.


## Assertions

### expectType&lt;T&gt;(value)

Check if a value is of a specific type.

### expectError(function)

Check if the function call has argument type errors.

### expectError&lt;T&gt;(value)

Check if a value is of the provided type `T`.


## License

MIT © [Sam Verschueren](https://github.com/SamVerschueren)