Add NonConstructor to allow customization of ObjectWrap behavior

doc/object_wrap.md

@@ -212,6 +212,29 @@ property of the `Napi::CallbackInfo`.
 
 Returns a `Napi::Function` representing the constructor function for the class.
 
+### OnCalledAsFunction
+
+Provides an opportunity to customize the behavior when a `Napi::ObjectWrap<T>`
+class is called from JavaScript as a function (without the **new** operator).
+
+The default behavior in this scenario is to throw a `Napi::TypeError` with the
+message `Class constructors cannot be invoked without 'new'`.  Define this
+public method on your derived class to override that behavior.
+
+For example, you could internally re-call the JavaScript contstructor _with_
+the **new** operator (via
+`Napi::Function::New(const std::vector<napi_value> &args)`), and return the
+resulting object.  Or you might do something else entirely, such as the way
+[`Date()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date#constructor)
+produces a string when called as a function.
+
+```cpp
+static Napi::Value OnCalledAsFunction(const Napi::CallbackInfo& callbackInfo);
+```
+
+- `[in] callbackInfo`: The object representing the components of the JavaScript
+request being made.
+
 ### Finalize
 
 Provides an opportunity to run cleanup code that requires access to the

napi-inl.h

@@ -4451,6 +4451,15 @@ inline ClassPropertyDescriptor<T> ObjectWrap<T>::StaticValue(Symbol name,
   return desc;
 }
 
+template <typename T>
+inline Value ObjectWrap<T>::OnCalledAsFunction(
+    const Napi::CallbackInfo& callbackInfo) {
+  NAPI_THROW(
+      TypeError::New(callbackInfo.Env(),
+                     "Class constructors cannot be invoked without 'new'"),
+      Napi::Value());
+}
+
 template <typename T>
 inline void ObjectWrap<T>::Finalize(Napi::Env /*env*/) {}
 
@@ -4464,8 +4473,8 @@ inline napi_value ObjectWrap<T>::ConstructorCallbackWrapper(
 
   bool isConstructCall = (new_target != nullptr);
   if (!isConstructCall) {
-    napi_throw_type_error(env, nullptr, "Class constructors cannot be invoked without 'new'");
-    return nullptr;
+    return details::WrapCallback(
+        [&] { return T::OnCalledAsFunction(CallbackInfo(env, info)); });
   }
 
   napi_value wrapper = details::WrapCallback([&] {

napi.h

@@ -2199,6 +2199,8 @@ namespace Napi {
     static PropertyDescriptor StaticValue(Symbol name,
                                           Napi::Value value,
                                           napi_property_attributes attributes = napi_default);
+    static Napi::Value OnCalledAsFunction(
+        const Napi::CallbackInfo& callbackInfo);
     virtual void Finalize(Napi::Env env);
 
   private:

test/binding.cc

@@ -65,6 +65,7 @@ Object InitTypedArray(Env env);
 Object InitGlobalObject(Env env);
 Object InitObjectWrap(Env env);
 Object InitObjectWrapConstructorException(Env env);
+Object InitObjectWrapFunction(Env env);
 Object InitObjectWrapRemoveWrap(Env env);
 Object InitObjectWrapMultipleInheritance(Env env);
 Object InitObjectReference(Env env);
@@ -152,6 +153,7 @@ Object Init(Env env, Object exports) {
   exports.Set("objectwrap", InitObjectWrap(env));
   exports.Set("objectwrapConstructorException",
       InitObjectWrapConstructorException(env));
+  exports.Set("objectwrap_function", InitObjectWrapFunction(env));
   exports.Set("objectwrap_removewrap", InitObjectWrapRemoveWrap(env));
   exports.Set("objectwrap_multiple_inheritance", InitObjectWrapMultipleInheritance(env));
   exports.Set("objectreference", InitObjectReference(env));

test/binding.gyp

@@ -66,6 +66,7 @@
         'typedarray.cc',
         'objectwrap.cc',
         'objectwrap_constructor_exception.cc',
+        'objectwrap_function.cc',
         'objectwrap_removewrap.cc',
         'objectwrap_multiple_inheritance.cc',
         'object_reference.cc',

test/objectwrap_function.cc

@@ -0,0 +1,45 @@
+#include <napi.h>
+#include <unordered_map>
+#include "test_helper.h"
+
+class FunctionTest : public Napi::ObjectWrap<FunctionTest> {
+ public:
+  FunctionTest(const Napi::CallbackInfo& info)
+      : Napi::ObjectWrap<FunctionTest>(info) {}
+
+  static Napi::Value OnCalledAsFunction(const Napi::CallbackInfo& info) {
+    // If called with a "true" argument, throw an exeption to test the handling.
+    if (!info[0].IsUndefined() && MaybeUnwrap(info[0].ToBoolean())) {
+      NAPI_THROW(Napi::Error::New(info.Env(), "an exception"), Napi::Value());
+    }
+    // Otherwise, act as a factory.
+    std::vector<napi_value> args;
+    for (size_t i = 0; i < info.Length(); i++) args.push_back(info[i]);
+    return MaybeUnwrap(GetConstructor(info.Env()).New(args));
+  }
+
+  // Constructor-per-env map in a static member because env.SetInstanceData()
+  // would interfere with Napi::Addon<T>
+  static std::unordered_map<napi_env, Napi::FunctionReference> constructors;
+
+  static void Initialize(Napi::Env env, Napi::Object exports) {
+    const char* name = "FunctionTest";
+    Napi::Function func = DefineClass(env, name, {});
+    constructors[env] = Napi::Persistent(func);
+    env.AddCleanupHook([env] { constructors.erase(env); });
+    exports.Set(name, func);
+  }
+
+  static Napi::Function GetConstructor(Napi::Env env) {
+    return constructors[env].Value();
+  }
+};
+
+std::unordered_map<napi_env, Napi::FunctionReference>
+    FunctionTest::constructors = {};
+
+Napi::Object InitObjectWrapFunction(Napi::Env env) {
+  Napi::Object exports = Napi::Object::New(env);
+  FunctionTest::Initialize(env, exports);
+  return exports;
+}

test/objectwrap_function.js

@@ -0,0 +1,22 @@
+'use strict';
+
+const assert = require('assert');
+const testUtil = require('./testUtil');
+
+function test (binding) {
+  return testUtil.runGCTests([
+    'objectwrap function',
+    () => {
+      const { FunctionTest } = binding.objectwrap_function;
+      const newConstructed = new FunctionTest();
+      const functionConstructed = FunctionTest();
+      assert(newConstructed instanceof FunctionTest);
+      assert(functionConstructed instanceof FunctionTest);
+      assert.throws(() => (FunctionTest(true)), /an exception/);
+    },
+    // Do on gc before returning.
+    () => {}
+  ]);
+}
+
+module.exports = require('./common').runTest(test);