# Jest (/docs/runtimes/jest)

`tyfigo/jest` is a Jest transformer. Add it for config files, and add their extensions to `moduleFileExtensions`:

```js title="jest.config.js"
/** @type {import('jest').Config} */
export default {
	testEnvironment: 'node',
	moduleFileExtensions: ['ts', 'js', 'json', 'yaml', 'yml', 'toml', 'jsonc', 'json5'],
	transform: {
		'\\.config\\.(yaml|yml|toml|jsonc|json5)$': 'tyfigo/jest',
		'\\.ts$': '@swc/jest',
	},
};
```

Add `ini`, `properties` and `xml` to both lists if you use those formats. For files in a
[`.config` directory](/docs/config-files#a-config-directory), use a pattern that matches both layouts:

```js
'(\\.config\\.|[\\\\/]\\.config[\\\\/][^\\\\/]+\\.)(yaml|yml|toml|jsonc|json5)$': 'tyfigo/jest',
```

Jest sets `NODE_ENV=test`, so `app.test.config.yaml` applies:

```yaml title="config/app.test.config.yaml"
tax:
  rate: 0.1
payments:
  provider: fake
```

```ts title="src/pricing.test.ts"
import { describe, expect, test } from '@jest/globals';

import config from '../config/app.config.yaml';

describe('config in Jest', () => {
	test('merges the test environment file and .env', () => {
		expect(config).toEqual({
			currency: 'USD',
			tax: { rate: 0.1, inclusive: false },
			payments: { provider: 'fake', apiKey: 'example-only-not-a-secret', timeoutMs: 250 },
		});
		expect(Object.isFrozen(config)).toBe(true);
	});
});
```

CommonJS tests (the default) get the config resolved when Jest transforms the file. Native ESM tests
(`NODE_OPTIONS=--experimental-vm-modules`) resolve it when the module is imported. The transformer takes `environment` and
`runtime` options: `['tyfigo/jest', { environment: 'staging' }]`.

See the [jest example](https://github.com/imranbarbhuiya/tyfigo/tree/main/examples/jest).