Jest | tyfigo

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

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, use a pattern that matches both layouts:

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

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

config/app.test.config.yaml

tax:
  rate: 0.1
payments:
  provider: fake

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.