Possible to add a cancel method to Promise in Typescript?

Question

I'd like to create a QueryPromise that is just a concrete promise with a cancel method. Here's how it would be used:

function runQuery(text: string): QueryPromise {
  return new QueryPromise((resolve,reject) => {nativeQuery(resolve)})
}

Here is my first attempt, hopefully pretty understandable:

interface CancellablePromise<T> extends Promise<T> {
  cancel: () => void
}

// create the promise my app will use
type QueryPromise = CancellablePromise<string|boolean>

But that's not sufficient.

After a few hours of trial and error I managed to get close, but this seems tedious and anything but DRY.

interface CancellablePromise<T> extends Promise<T> {
  cancel: () => void
}
// also need this interface so the variable can be declared
// plus need to redeclare methods to return the new type
interface CancellablePromiseConstructor extends PromiseConstructor {
  new <T>(executor: (resolve: (value?: T | PromiseLike<T>) => void, reject: (reason?: any) => void) => void): CancellablePromise<T>;
  cancel: () => void
}

type QueryPromise = CancellablePromise<string|boolean>    // for parameter types
var QueryPromise: CancellablePromiseConstructor = Promise // for the new operator

// some example code to exercise the promise
function runQuery(text: string): QueryPromise {
  return new QueryPromise((resolve,reject) => {nativeQuery(resolve)})
}

I feel like I've gone a long way down the wrong road... Does anyone know of a better way to do this?

Answer 1

TypeScript interfaces and types describe contracts. Yours are fine:

interface CancellablePromise<T> extends Promise<T> {
  cancel: () => void
}

type QueryPromise = CancellablePromise<string | boolean>

You can then implement a contract as you want. Here is an example:

function runQuery(text: string): QueryPromise {
  let rejectCb: (err: Error) => void
  let p: Partial<QueryPromise> = new Promise<string | boolean>((resolve, reject) => {
    rejectCb = reject
    /* ... Here the implementation of the query ... */
  });
  p.cancel = () => {
    /* ... Here the implementation of the aborting ... */
    rejectCb(new Error("Canceled"))
  }
  return p as QueryPromise
}

Notices:

• The implementation of cancel should reject the promise;
• I use Partial<QueryPromise> in order to add the member cancel afterward.