Address PR review comments

- Add test for virtual module as entrypoint (requested by alii)
- Include module name in error messages for better debugging (requested by alii)
- Check outputs in bun-build-api test to verify virtual module content (requested by alii)
- Use intrinsic argument validation (, ) for consistency
- Guard against undefined onResolve map to prevent crashes

All tests continue to pass with these improvements.

src/js/builtins/BundlerPlugin.ts

@@ -356,11 +356,11 @@ export function runSetupFunction(
     onStart,
     resolve: notImplementedIssueFn(2771, "build.resolve()"),
     module: (specifier: string, callback: () => { contents: string; loader?: string }) => {
-      if (typeof specifier !== "string") {
-        throw new TypeError("module() specifier must be a string");
+      if (!(typeof specifier === "string")) {
+        throw $ERR_INVALID_ARG_TYPE("specifier", "string", specifier);
       }
-      if (typeof callback !== "function") {
-        throw new TypeError("module() callback must be a function");
+      if (!$isCallable(callback)) {
+        throw $ERR_INVALID_ARG_TYPE("callback", "function", callback);
       }
 
       // Store the virtual module
@@ -428,6 +428,11 @@ export function runOnResolvePlugins(this: BundlerPlugin, specifier, inputNamespa
       return null;
     }
 
+    if (!onResolve) {
+      this.onResolveAsync(internalID, null, null, null);
+      return null;
+    }
+
     var results = onResolve.$get(inputNamespace);
     if (!results) {
       this.onResolveAsync(internalID, null, null, null);
@@ -558,22 +563,22 @@ export function runOnLoadPlugins(
 
         try {
           if (!result || !$isObject(result)) {
-            throw new TypeError('Virtual module must return an object with "contents" property');
+            throw new TypeError(`Virtual module "${path}" must return an object with "contents" property`);
           }
 
           var { contents, loader = "js" } = result;
 
           if (!(typeof contents === "string")) {
-            throw new TypeError('Virtual module must return an object with "contents" as a string');
+            throw new TypeError(`Virtual module "${path}" must return an object with "contents" as a string`);
           }
 
           if (!(typeof loader === "string")) {
-            throw new TypeError('Virtual module "loader" must be a string if provided');
+            throw new TypeError(`Virtual module "${path}" "loader" must be a string if provided`);
           }
 
           const chosenLoader = LOADERS_MAP[loader];
           if (chosenLoader === undefined) {
-            throw new TypeError(`Loader ${loader} is not supported.`);
+            throw new TypeError(`Virtual module "${path}": Loader ${loader} is not supported.`);
           }
 
           this.onLoadAsync(internalID, contents, chosenLoader);

test/bundler/bun-build-api.test.ts

@@ -424,6 +424,11 @@ describe("Bun.build", () => {
     });
 
     expect(result.success).toBe(true);
+    expect(result.outputs).toHaveLength(1);
+    
+    // Check that the virtual module content is in the output
+    const output = await result.outputs[0].text();
+    expect(output).toContain("Hello from virtual module");
   });
 
   test("non-object plugins throw invalid argument errors", () => {

test/bundler/bundler-plugin-virtual-modules.test.ts

@@ -406,6 +406,32 @@ test("Bun.build plugin virtual modules - onLoad plugins still work", async () =>
   expect(output).toContain("by onLoad plugin");
 });
 
+test("Bun.build plugin virtual modules - virtual module as entrypoint", async () => {
+  using dir = tempDir("virtual-entrypoint", {});
+
+  const result = await Bun.build({
+    entrypoints: ["virtual-entry"],
+    outdir: String(dir),
+    plugins: [
+      {
+        name: "in-memory-entrypoint",
+        setup(build) {
+          build.module("virtual-entry", () => ({
+            contents: `console.log("Hello from virtual entrypoint");`,
+            loader: "js",
+          }));
+        },
+      },
+    ],
+  });
+
+  expect(result.success).toBe(true);
+  expect(result.outputs).toHaveLength(1);
+  
+  const output = await Bun.file(result.outputs[0].path).text();
+  expect(output).toContain("Hello from virtual entrypoint");
+});
+
 test("Bun.build plugin virtual modules - no memory leak on repeated builds", async () => {
   using dir = tempDir("virtual-memory", {
     "entry.ts": `