fix(bun:test): return an object when a mock function is constructed

src/jsc/bindings/JSMockFunction.cpp

@@ -86,6 +86,7 @@ inline To tryJSDynamicCast(JSC::WriteBarrier<WriteBarrierT>& from)
 }
 
 JSC_DECLARE_HOST_FUNCTION(jsMockFunctionCall);
+JSC_DECLARE_HOST_FUNCTION(jsMockFunctionConstruct);
 JSC_DECLARE_CUSTOM_GETTER(jsMockFunctionGetter_protoImpl);
 JSC_DECLARE_CUSTOM_GETTER(jsMockFunctionGetter_mock);
 JSC_DECLARE_HOST_FUNCTION(jsMockFunctionGetter_mockGetLastCall);
@@ -462,7 +463,7 @@ class JSMockFunction : public JSC::InternalFunction {
     }
 
     JSMockFunction(JSC::VM& vm, JSC::Structure* structure, CallbackKind wrapKind)
-        : Base(vm, structure, jsMockFunctionCall, jsMockFunctionCall)
+        : Base(vm, structure, jsMockFunctionCall, jsMockFunctionConstruct)
     {
         initMock();
     }
@@ -826,7 +827,7 @@ static JSValue createMockResult(JSC::VM& vm, Zig::GlobalObject* globalObject, co
     return result;
 }
 
-JSC_DEFINE_HOST_FUNCTION(jsMockFunctionCall, (JSGlobalObject * lexicalGlobalObject, CallFrame* callframe))
+static JSC::EncodedJSValue jsMockFunctionCallImpl(JSGlobalObject* lexicalGlobalObject, CallFrame* callframe, JSValue thisValue)
 {
     Zig::GlobalObject* globalObject = uncheckedDowncast<Zig::GlobalObject>(lexicalGlobalObject);
     auto& vm = JSC::getVM(globalObject);
@@ -838,7 +839,6 @@ JSC_DEFINE_HOST_FUNCTION(jsMockFunctionCall, (JSGlobalObject * lexicalGlobalObje
     }
 
     JSC::ArgList args = JSC::ArgList(callframe);
-    JSValue thisValue = callframe->thisValue();
     JSC::JSArray* argumentsArray = nullptr;
     {
         JSC::ObjectInitializationScope object(vm);
@@ -879,6 +879,20 @@ JSC_DEFINE_HOST_FUNCTION(jsMockFunctionCall, (JSGlobalObject * lexicalGlobalObje
         fn->contexts.set(vm, fn, contexts);
     }
 
+    JSC::JSArray* instances = fn->instances.get();
+    if (instances) {
+        instances->push(globalObject, thisValue);
+        RETURN_IF_EXCEPTION(scope, {});
+    } else {
+        JSC::ObjectInitializationScope object(vm);
+        instances = JSC::JSArray::tryCreateUninitializedRestricted(
+            object,
+            globalObject->arrayStructureForIndexingTypeDuringAllocation(JSC::ArrayWithContiguous),
+            1);
+        instances->initializeIndex(object, 0, thisValue);
+        fn->instances.set(vm, fn, instances);
+    }
+
     auto invocationId = JSMockModule::nextInvocationId();
     JSC::JSArray* invocationCallOrder = fn->invocationCallOrder.get();
     if (invocationCallOrder) {
@@ -981,6 +995,36 @@ JSC_DEFINE_HOST_FUNCTION(jsMockFunctionCall, (JSGlobalObject * lexicalGlobalObje
     return JSValue::encode(jsUndefined());
 }
 
+JSC_DEFINE_HOST_FUNCTION(jsMockFunctionCall, (JSGlobalObject * lexicalGlobalObject, CallFrame* callframe))
+{
+    return jsMockFunctionCallImpl(lexicalGlobalObject, callframe, callframe->thisValue());
+}
+
+JSC_DEFINE_HOST_FUNCTION(jsMockFunctionConstruct, (JSGlobalObject * lexicalGlobalObject, CallFrame* callframe))
+{
+    auto& vm = JSC::getVM(lexicalGlobalObject);
+    auto scope = DECLARE_THROW_SCOPE(vm);
+
+    // A native construct callback must return an object, so build `this` from
+    // newTarget's prototype and fall back to it when the mock returns a non-object.
+    JSObject* newTarget = asObject(callframe->newTarget());
+    JSValue prototype = newTarget->get(lexicalGlobalObject, vm.propertyNames->prototype);
+    RETURN_IF_EXCEPTION(scope, {});
+
+    JSObject* thisObject = prototype.isObject()
+        ? JSC::constructEmptyObject(lexicalGlobalObject, asObject(prototype))
+        : JSC::constructEmptyObject(lexicalGlobalObject);
+
+    JSValue returnValue = JSValue::decode(jsMockFunctionCallImpl(lexicalGlobalObject, callframe, thisObject));
+    RETURN_IF_EXCEPTION(scope, {});
+
+    if (returnValue && returnValue.isObject()) {
+        return JSValue::encode(returnValue);
+    }
+
+    return JSValue::encode(thisObject);
+}
+
 void JSMockFunctionPrototype::finishCreation(JSC::VM& vm, JSC::JSGlobalObject* globalObject)
 {
     Base::finishCreation(vm);

test/js/bun/test/mock-fn.test.js

@@ -117,6 +117,60 @@ describe("mock()", () => {
     expect(fn).toHaveBeenCalledWith();
   });
 
+  test("are constructable with new", () => {
+    // no implementation: `new` should produce a fresh object, like `new` on an ordinary function
+    const fn = jest.fn();
+    const instance = new fn(1, 2);
+    expect(typeof instance).toBe("object");
+    expect(instance).not.toBe(null);
+    expect(fn.mock.calls).toEqual([[1, 2]]);
+    expect(fn.mock.contexts[0]).toBe(instance);
+    // `new` calls are recorded in mock.instances
+    expect(fn.mock.instances[0]).toBe(instance);
+
+    // Reflect.construct used to crash when the mock returned a non-object
+    const reflected = Reflect.construct(fn, []);
+    expect(typeof reflected).toBe("object");
+    expect(fn.mock.instances[1]).toBe(reflected);
+
+    // implementation operating on `this`
+    const withImpl = jest.fn(function (value) {
+      this.value = value;
+    });
+    const constructed = new withImpl(42);
+    expect(constructed.value).toBe(42);
+    expect(withImpl.mock.contexts[0]).toBe(constructed);
+    expect(withImpl.mock.instances[0]).toBe(constructed);
+
+    // implementation returning an object wins over the created `this`
+    const returnsObject = jest.fn(() => ({ a: 1 }));
+    expect(new returnsObject()).toEqual({ a: 1 });
+
+    // primitive return values are ignored by `new`, like ordinary functions
+    const returnsPrimitive = jest.fn().mockReturnValue(42);
+    expect(typeof new returnsPrimitive()).toBe("object");
+
+    // newTarget.prototype is respected
+    const classLike = jest.fn();
+    classLike.prototype = {
+      greet() {
+        return "hello";
+      },
+    };
+    const classInstance = new classLike();
+    expect(classInstance.greet()).toBe("hello");
+    expect(classInstance instanceof classLike).toBe(true);
+
+    // Reflect.construct with an explicit newTarget honors its prototype
+    class Base {}
+    expect(Reflect.construct(jest.fn(), [], Base)).toBeInstanceOf(Base);
+
+    // a normal call with a non-undefined `this` must still return the mock's value
+    const method = jest.fn().mockReturnValue(42);
+    const host = { method };
+    expect(host.method()).toBe(42);
+  });
+
   test("mockName returns this", () => {
     const fn = jest.fn();
     expect(fn.mockName()).toBe(fn);