Differences from the Spec

Where JavaScript can't do what the spec says, or where the spec text looks like an oversight, and what this implementation does instead.

Where JavaScript can't do what the spec says, or where the spec text looks like an oversight, this implementation does something else:

• Memory is never reclaimed. Shared objects and interned strings live until the process ends. There is no collector across agents.
• Symbols. Only registered (Symbol.for) and well-known symbols can be shared, as values or as field keys, because other agents don't have the others. Reflect.canBeShared returns false for the rest.
• [[GetOwnProperty]] reports shared fields as writable. The spec says not writable, but its own [[DefineOwnProperty]] asserts writable, and a Proxy can't report a writable target property as non-writable.
• Constructors are classes. A shared struct's constructor has a prototype object (with a null prototype; its instances still have none). A struct with no extends has a hidden root function as its constructor's [[Prototype]], not Function.prototype.
• Arguments. The spec evaluates the arguments of new S(...) and of super(...) but never passes them to the initializer. Here they are passed.
• Methods are methods. The spec wraps each instance method in a struct method exotic object, which checks this and has no own properties. Here the check is the method's first statement, so it keeps its name and length. A method whose parameters run code (a default, a destructuring) is called through another that checks first.
• Getters and setters check this too. The spec checks only methods: its DefineMethod makes no struct method of an accessor. Here an instance getter or setter throws a TypeError on what is not an instance of its struct, as a method does.
• Static methods are not brand-checked. The spec's DefineMethod would wrap them too, which would make them impossible to call.
• super.x in a shared struct's initializers looks at the parent constructor. In the spec their home object is null, and super.x throws.
• Mutexes aren't fair. A woken waiter competes with newcomers, where the spec has a waiter queue.