Support require() in config files

docs/06-configuration.md

@@ -69,8 +69,9 @@ Note that providing files on the CLI overrides the `files` option. If you've con
 
 To use an `ava.config.js` file:
 
- 1. It must be in the same directory as your `package.json`
- 2. Your `package.json` must not contain an `ava` property (or, if it does, it must be an empty object)
+1. It must be in the same directory as your `package.json`
+2. Your `package.json` must not contain an `ava` property (or, if it does, it must be an empty object)
+3. You must use `export default`, though [`require()`](https://nodejs.org/api/modules.html#modules_require_id) is available to load non-ES modules
 
 The config file must have a default export, using ES modules. It can either be a plain object or a factory function which returns a plain object:
 

lib/load-config.js

@@ -15,7 +15,15 @@ function loadConfig(defaults = {}) {
 	let fileConf;
 	try {
 		({default: fileConf = MISSING_DEFAULT_EXPORT} = esm(module, {
-			cjs: false,
+			cjs: {
+				cache: false,
+				extensions: false,
+				interop: false,
+				mutableNamespace: false,
+				namedExports: false,
+				paths: false,
+				vars: true
+			},
 			force: true,
 			mode: 'all'
 		})(path.join(projectDir, 'ava.config.js')));

test/fixture/load-config/require/ava.config.js

@@ -0,0 +1,3 @@
+const config = require('./cjs');
+
+export default config;

test/fixture/load-config/require/cjs.js

@@ -0,0 +1,3 @@
+module.exports = {
+	files: 'config-file-cjs-test-value'
+};

test/fixture/load-config/require/package.json

@@ -0,0 +1 @@
+{}

test/load-config.js

@@ -52,6 +52,13 @@ test('loads config from factory function', t => {
 	t.end();
 });
 
+test('supports require() inside config file', t => {
+	changeDir('require');
+	const conf = loadConfig();
+	t.is(conf.files, 'config-file-cjs-test-value');
+	t.end();
+});
+
 test('throws an error if a config factory returns a promise', t => {
 	changeDir('factory-no-promise-return');
 	t.throws(loadConfig);