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JavaScript values are represented using ValueWrapper class template and its subclasses. The template argument tells whether the value should be freed upon the destruction of the wrapper. Thus, for convenience, there are type aliases like the following for all value types:

  • using Value = ValueWrapper<true> - a wrapper for a value that should be freed (strong reference)
  • using ValueWeak = ValueWrapper<false> - a wrapper for a value that should not be freed (weak reference)

Value has ownership of the value (reference), whereas ValueWeak does not, meaning the garbage collector can free the value referenced by a ValueWeak at any time and render the wrapper invalid.

Value types

  • Value - plain value, can contain any type including undefined and null
  • Object
  • Function
  • Array
  • Promise
  • ArrayBuffer
  • Exception - either a JavaScript value or an Error template, which can be propagated to the JavaScript

API documentation for the value types can be found in the API documentation.

Creating a JavaScript value

New values can be created using the static methods of the wrapper classes.

For exampleValue::from<T>(ContextRef, T) will convert the value to a JavaScript value (if a ConvTraits specialization exists for the type T)

Value value = Value::from(machine.context(), 42);

A new object can be created using Object::create(ContextRef) method.

Object obj = Object::create(machine.context());

Converting a JavaScript value to a C++ type

Values can be converted to C++ types using Value::to<T>() method, which will convert the value to a C++ type. There must exist a ConvTraits specialization for the type T.

int i =<int>();

Note that this conversion can be used to convert the value to a Value subclass, such as Object, Function, or Exception.

A list of default conversion traits can be found in traits.h

Defining a custom conversion trait

To define a conversion trait for a custom type, simply define a specialization of the ConvTraits template for the type.

struct ConvTraits<MyType> {
    static MyType from(ContextRef ctx, ValueWeak val) {
        // ...

    static Value to(ContextRef ctx, MyType value) {
        // ...