Add support for using declaration

[Flarna]

Search Terms

using-declaration, using, declaration, overload

Suggestion

Add language support similar to Using-declaration in C++ (https://en.cppreference.com/w/cpp/language/using_declaration) to pull in names from base class.

Use Cases

In derived class a new overload is added to an API. This requires that overloads from base classes are copied to derived class otherwise typescript complains. A keyword like the using-declaration in C++ to pull in names from base class would be handy to avoid duplication.

Examples

class Emitter {
  addListener(event: "foo", cb: (p: string) => void): void;
  addListener(event: "bar", cb: (p: string, e: number) => void): void;
  addListener(event: string, cb: (...arg: any[]) => void): void {}
}

class SpecialEmitter extends Emitter {
  // I would prefer using Emitter.addListener or similar here
  addListener(event: "foo", cb: (p: string) => void): void;
  addListener(event: "bar", cb: (p: string, e: number) => void): void;

  // new overloads supported by SpecialEmitter
  addListener(event: "new", cb: (x: RegExp) => void): void;
  addListener(event: "next", cb: (x: number, b: boolean) => void): void;
  addListener(event: string, cb: (...arg: any[]) => void): void {}
}

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.