typeof class

[DanielRosenwasser]

@ahejlsberg has had a few experiments to try to make it easier to describe the type of a class. Specifically:

• avoiding the awkward static/instance pattern that users need to write on anonymous object types/interface types

  var Thing: {
    new () => {
      instanceField: number;
    }
    staticField: string;
  }

• allowing users to describe types with class-specific characteristics (e.g. abstract)

The idea is to allow typeof to be followed by a class expression.

var Thing: typeof class Thing {
  static staticField: string;

  instanceField: number;
  constructor() {}
}

There are definitely some subtleties here. For example, handling loops and whether this syntax is allowed in JSDoc with a TypeScript-annotated class:

/** @type { typeof class C<T> { x: T } } */
//                        ^^^    ^^^
// TypeScript type parameters and annotations
// Are these allowed?
var Thing;

[ExE-Boss]

Would this make it possible to revisit microsoft/TypeScript-DOM-lib-generator#858 and #39504 (comment)?

[jakebailey]

This is coming up again for isolated declarations; what is preventing us from emitting for class expressions in d.ts initializers, then referencing them that way?

export const a = class A {
    readonly a: A = new A();
}

// emits as

export const a = class A {
    readonly a: A;
}