bun:test: fix crash when constructing a mock function via Reflect.construct

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();
     }
@@ -981,6 +982,35 @@ JSC_DEFINE_HOST_FUNCTION(jsMockFunctionCall, (JSGlobalObject * lexicalGlobalObje
     return JSValue::encode(jsUndefined());
 }
 
+JSC_DEFINE_HOST_FUNCTION(jsMockFunctionConstruct, (JSGlobalObject * globalObject, CallFrame* callframe))
+{
+    auto& vm = JSC::getVM(globalObject);
+    auto scope = DECLARE_THROW_SCOPE(vm);
+
+    // [[Construct]] must return an object, but the mock implementation can return any value.
+    // Mirror ordinary JS constructor semantics: create `this` from new.target's prototype,
+    // invoke the mock with it, and fall back to it when the implementation returns a primitive.
+    JSValue newTarget = callframe->newTarget();
+    JSObject* thisObject = nullptr;
+    if (newTarget.isObject()) [[likely]] {
+        JSValue prototype = asObject(newTarget)->get(globalObject, vm.propertyNames->prototype);
+        RETURN_IF_EXCEPTION(scope, {});
+        if (prototype.isObject())
+            thisObject = JSC::constructEmptyObject(globalObject, asObject(prototype));
+    }
+    if (!thisObject)
+        thisObject = JSC::constructEmptyObject(globalObject);
+
+    callframe->setThisValue(thisObject);
+    JSValue result = JSValue::decode(jsMockFunctionCall(globalObject, callframe));
+    RETURN_IF_EXCEPTION(scope, {});
+
+    if (result && result.isObject())
+        return JSValue::encode(result);
+
+    return JSValue::encode(thisObject);
+}
+
 void JSMockFunctionPrototype::finishCreation(JSC::VM& vm, JSC::JSGlobalObject* globalObject)
 {
     Base::finishCreation(vm);

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

@@ -309,6 +309,39 @@ describe("mock()", () => {
     expect(fn).toHaveBeenLastCalledWith();
     expect(fn).toHaveBeenCalledWith();
   });
+  test("new works", () => {
+    const fn = jest.fn(function () {
+      this.a = 1;
+    });
+    const instance = new fn();
+    expect(typeof instance).toBe("object");
+    expect(instance.a).toBe(1);
+    expect(fn).toHaveBeenCalledTimes(1);
+    expect(fn.mock.contexts[0]).toBe(instance);
+
+    // an object returned from the implementation becomes the result of `new`
+    const returned = { b: 2 };
+    const fn2 = jest.fn(() => returned);
+    expect(new fn2()).toBe(returned);
+
+    // a primitive return value is ignored, like ordinary constructors
+    const fn3 = jest.fn(() => 42);
+    expect(typeof new fn3()).toBe("object");
+
+    // same for mockReturnValue
+    const fn4 = jest.fn().mockReturnValue(5);
+    expect(typeof new fn4()).toBe("object");
+
+    // Reflect.construct on a mock with no implementation returns an object
+    const fn5 = jest.fn();
+    expect(typeof Reflect.construct(fn5, [])).toBe("object");
+    expect(fn5).toHaveBeenCalledTimes(1);
+
+    // the prototype of the instance comes from the mock's .prototype
+    const fn6 = jest.fn();
+    fn6.prototype = { marker: 3 };
+    expect(new fn6().marker).toBe(3);
+  });
   test(".name works", () => {
     const fn = jest.fn(function hey() {
       return this;
@@ -828,6 +861,19 @@ describe("spyOn", () => {
     expect(fn).not.toHaveBeenCalled();
   });
 
+  test("constructing a spy works", () => {
+    var obj = {
+      Original: function () {
+        this.ok = true;
+      },
+    };
+    const fn = spyOn(obj, "Original");
+    const instance = Reflect.construct(obj.Original, []);
+    expect(typeof instance).toBe("object");
+    expect(instance.ok).toBe(true);
+    expect(fn).toHaveBeenCalledTimes(1);
+  });
+
   test("override impl after doesnt break restore", () => {
     var obj = {
       original() {