How to fix Type 'this' is not a class type in TypeScript

Create a reusable constructor alias and use it to constrain the base class rather than the instance this:

type Constructor<T = {}> = new (...args: any[]) => T;

function Timestamped<TBase extends Constructor>(Base: TBase) {
  return class extends Base {
    timestamp = new Date();
  };
}

This keeps TBase on the constructor side, so the mixin never mentions the instance this in a place that expects a class type.

Static methods can see the constructor by typing this as T extends typeof Base:

class Base {
  static make<T extends typeof Base>(this: T) {
    return new this();
  }
}

Here this is still polymorphic, but the constraint T extends typeof Base keeps the compiler on the constructor side and avoids TS2509.

Reserve the instance this for methods, accessors, or this parameters so the compiler always knows it describes the subtype:

class Fluent {
  setName(this: this, name: string) {
    return Object.assign(this, { name });
  }
}

Do not try to reuse that same this type for extends, implements, or new clauses; those positions need constructor-like types instead.