Fall back to default .js handler for unknown extensions

Co-authored-by: Mark Wubben <[email protected]>

Author: gloryofrobots

fixture.cjs

@@ -0,0 +1 @@
+module.exports = 'foobar-cjs';

index.js

@@ -1,13 +1,18 @@
 'use strict';
 module.exports = install;
 
-function install(precompile, ext = '.js', extensions = require.extensions) { // eslint-disable-line node/no-deprecated-api
-	const {[ext]: oldExtension} = extensions;
+/* eslint-disable node/no-deprecated-api, eslint-comments/disable-enable-pair */
+
+// Save reference before a new handler is installed.
+const {'.js': defaultHandler} = require.extensions;
+
+function install(precompile, ext = '.js', extensions = require.extensions) {
+	const {[ext]: fallbackHandler = defaultHandler} = extensions;
 
 	extensions[ext] = function (module, filename) {
 		const source = precompile(filename);
 		if (source === null) {
-			Reflect.apply(oldExtension, extensions, [module, filename]);
+			Reflect.apply(fallbackHandler, extensions, [module, filename]);
 		} else {
 			module._compile(source, filename);
 		}

test.js

@@ -64,3 +64,12 @@ test('test actual require', t => {
 
 	t.is(require('./fixture'), 'foobar');
 });
+
+test('test require extension without previous handler extension', t => {
+	install(
+		() => null,
+		'.cjs'
+	);
+
+	t.is(require('./fixture.cjs'), 'foobar-cjs');
+});