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Classes, in the context of Jaculus-machine are, similarly to JavaScript classes, templates for creating objects. Unlike raw JavaScript classes, however, classes defined in C++ can contain opaque C++ data, which can then be used through an interface defined in the class definition.

Defining a class

To create a class, you must first define a ProtoBuilder structure. A ProtoBuilder describes the class's behavior and prototype through a static interface. Features of the class are specified by inheriting from the structures in the jac::ProtoBuilder namespace and overriding their static interfaces. These structs contain a default implementation of their interface and some convenience functions for describing the class.

The following base structs are available (with all of their interface methods):

using namespace jac;

struct MyBuilder : public ProtoBuilder::Properties {
    static void addProperties(ContextRef ctx, Object proto) {
        proto.defineProperty("foo", Value::from(ctx, 42));

struct MyBuilder2 : public ProtoBuilder::Callable {
    static Value callFunction(ContextRef ctx, ValueWeak funcObj, ValueWeak thisVal, std::vector<ValueWeak> args) {
        return Value::from(ctx, static_cast<int>(args.size()));

    // the default implementation - does not have to be overridden if not needed
    static Value callConstructor(ContextRef ctx, ValueWeak funcObj, ValueWeak target, std::vector<ValueWeak> args) {
        throw Exception::create(Exception::Type::TypeError, "Class cannot be called as a constructor");

struct MyBuilder3 : public ProtoBuilder::Opaque<MyClass>, public ProtoBuilder::Properties {
    static MyClass* constructOpaque(ContextRef ctx, std::vector<ValueWeak> args) {
        return new MyClass();

    // the default implementation - does not have to be overridden if not needed
    static void destructOpaque(JSRuntime* rt, MyClass* ptr) {
        delete ptr;

    static void addProperties(ContextRef ctx, Object proto) {
        addPropMember<int, &MyClass::foo>(ctx, proto, "foo");

        FunctionFactory ff(ctx);
        proto.defineProperty("bar", ff.newFunctionThis([](ContextRef ctx, ValueWeak thisValue) {
            // get the opaque pointer from the thisValue
            MyClass& self = *getOpaque(ctx, thisValue);

The jac::Class template can be instantiated with the ProtoBuilder struct to create a class definition, and a name can be assigned using the init method. The init method can be called repeatedly without any effect if called with the same name; otherwise, an exception will be thrown.

using MyClassJs = Class<MyBuilder>;


To use the class in a given Context, the class must be initialized in the Context by calling the initContext method:

ContextRef ctx = ...;


After the class and context are initialized, you can get the class constructor and prototype and instantiate the class:

Value constructor = MyClassJs::getConstructor(ctx);
Value prototype = MyClassJs::getPrototype(ctx);

Value obj =<Function>().callConstructor();