fix: strip BOM before calling transform hook (#5268)

src/Module.ts

@@ -810,10 +810,6 @@ export default class Module {
 			const shebangEndPosition = code.indexOf('\n');
 			this.shebang = code.slice(2, shebangEndPosition);
 		}
-		/* eslint-disable-next-line unicorn/number-literal-case */
-		if (code.charCodeAt(0) === 0xfe_ff) {
-			code = code.slice(1);
-		}
 
 		timeStart('generate ast', 3);
 

src/ModuleLoader.ts

@@ -296,6 +296,11 @@ export class ModuleLoader {
 				: source != null && typeof source === 'object' && typeof source.code === 'string'
 				  ? source
 				  : error(logBadLoader(id));
+		const code = sourceDescription.code;
+		/* eslint-disable-next-line unicorn/number-literal-case */
+		if (code.charCodeAt(0) === 0xfe_ff) {
+			sourceDescription.code = code.slice(1);
+		}
 		const cachedModule = this.graph.cachedModules.get(id);
 		if (
 			cachedModule &&

test/function/samples/strip-bom-1/_config.js

@@ -0,0 +1,8 @@
+const commonjs = require('@rollup/plugin-commonjs');
+
+module.exports = defineTest({
+	description: 'Works correctly with BOM files and the @rollup/plugin-commonjs plugin.',
+	options: {
+		plugins: [commonjs()]
+	}
+});

test/function/samples/strip-bom-1/main.js

@@ -0,0 +1,12 @@
+(function () {
+	const foo = 'foo';
+	if (typeof define === 'function' && define.amd) {
+		define([], function () {
+			return foo;
+		});
+	} else if (typeof module !== 'undefined' && module.exports) {
+		module.exports = foo;
+	} else {
+		this.foo = foo;
+	}
+})();