fix(plugins): onLoad returning CJS contents works with require()

When a plugin's onLoad handler returned CommonJS-style contents
(module.exports), the transpiler correctly detected CJS and wrapped
the code, but handleVirtualModuleResult always routed through the ESM
loader, ignoring the isCommonJSModule flag. The CJS wrapper function
was loaded as ESM where it was never called, resulting in empty exports.

Now check isCommonJSModule after transpiling virtual module contents
and evaluate through the CJS path when require() is the consumer.

Closes #27799

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/bun.js/bindings/ModuleLoader.cpp

@@ -390,6 +390,13 @@ static JSValue handleVirtualModuleResult(
             RELEASE_AND_RETURN(scope, reject(JSValue::decode(res->result.err.value)));
         }
 
+        if (commonJSModule && res->result.value.isCommonJSModule) {
+            auto specifierString = specifier->toWTFString(BunString::ZeroCopy);
+            commonJSModule->evaluate(globalObject, specifierString, res->result.value);
+            RETURN_IF_EXCEPTION(scope, {});
+            return commonJSModule;
+        }
+
         auto provider = Zig::SourceProvider::create(globalObject, res->result.value);
         return resolve(JSC::JSSourceCode::create(vm, JSC::SourceCode(provider)));
     }

test/regression/issue/27799.test.ts

@@ -0,0 +1,50 @@
+import { expect, test } from "bun:test";
+import { bunEnv, bunExe, tempDir } from "harness";
+
+test("onLoad plugin returning CJS contents works with require()", async () => {
+  using dir = tempDir("issue-27799", {
+    "plugin.js": `
+Bun.plugin({
+  name: "cjs-patch",
+  setup(build) {
+    build.onLoad({ filter: /tiny-module\\.js$/ }, () => {
+      return {
+        loader: "js",
+        contents: \`
+module.exports.greet = function greet(name) { return "patched hello " + name; }
+module.exports.add = function add(a, b) { return a + b + a; }
+\`,
+      };
+    });
+  },
+});
+`,
+    "tiny-module.js": `
+module.exports.greet = function greet(name) {
+    return "hello " + name;
+}
+module.exports.add = function add(a, b) {
+    return a + b;
+}
+`,
+    "run-demo.js": `
+const { greet, add } = require('./tiny-module');
+console.log(greet('world'));
+console.log(add(1, 2));
+`,
+  });
+
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "--preload=./plugin.js", "run-demo.js"],
+    env: bunEnv,
+    cwd: String(dir),
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+  expect(stdout).toBe("patched hello world\n4\n");
+  expect(stderr).toBe("");
+  expect(exitCode).toBe(0);
+});