Structs

A struct is a class with a different construction: fixed-layout, sealed, made in one shot, with methods that check their receiver.

A struct is a class with a different construction. The compiler keeps the class, which gives the struct its scope, private names and static elements. It moves the fields and the constructor into the runtime's hands (src/runtime/structs.ts shows the shape). An instance is made the spec's way (§1.1.6):

1. All of its fields, the base struct's first, are defined as undefined, and it is sealed.
2. The field initializers run, from the base struct down.
3. The constructor runs. It is an initializer, and super(...) calls the parent struct's initializer.

Instance methods check that this is an instance of their struct (struct method exotic objects, §1.2), and so do getters and setters here. Constructing anything but the struct itself is a TypeError, so a class cannot extend a struct.