[Feature Request] A way to instruct how to infer this

[bigopon]

TypeScript Version: 2.3.2

Code

I have this definition:

extend<T, K>(cls: (...args: {}[]) => T, xtn: K): new (...args: {}[]) => T & K

Usage:

var NewClass = extend(MyClass, {
  init(a) {
    this.doSomething();
  },

  // ....
})

The NewClass will still have all expected method init ..., but the this cannot be inferred inside the extension object.

It would be great if there is a way to instruct Typescript how to infer the this inside extension object based on the first passed param meter. (MyClass in the example). The reason is to have better intellisense.

[DanielRosenwasser]

We actually shipped this in TypeScript 2.3 to support APIs like those in Ember and Vue.

See #14141.

[DanielRosenwasser]

For the record, you probably want

extend<T, K>(cls: (...args: {}[]) => T, xtn: K & ThisType<T & K>): new (...args: {}[]) => T & K

[bigopon]

It looks awesome. But I couldn't make it work. Tried adding noImplicitThis to the jsconfig but i got warning that it is not a valid config.

And makes me wonder if the following is doable

interface Observable {
  /**
   * Very long long long comment to explain how to use `options`
   */
  on(event: string, handler: Function, options?: object): Observable
}

interface MyClass extends Observable {
  // ...
}

We know that using new MyClass().on(...) will return MyClass instance, but how to tell Typescript to return MyClass instead of Observable. I don't wanna rewrite all the comment on the overload method on MyClass interface.

Edit: My fault on some interface declaration. it works perfect.

[kitsonk]

interface Observable {
  /**
   * Very long long long comment to explain how to use `options`
   */
  on(event: string, handler: Function, options?: object): this
}

interface MyClass extends Observable {
  // ...
}