Typescript Promise rejection type

How do I set the type of the rejection of my promise? Let's say I do:

const start = (): Promise<string> => {
   return new Promise((resolve, reject) => {
      if (someCondition) {
         resolve('correct!');
      } else {
         reject(-1);
      }
   });
}

Let's say I want to reject with a number. But I cannot set the type; I can pass whatever I want to the reject here.

Moreover, when using this promise, I want to have compiling error if I use the rejection response type incorrectly.

As explained in this issue, Promise doesn't have different types for fulfilled and rejected promises. reject accepts any argument that doesn't affect type of a promise.

Currently Promise cannot be typed any better. This results from the fact that a promise can be rejected by throwing inside then or catch (this is a preferable way to reject existing promise), and this cannot be handled by typing system; also, TypeScript also doesn't have exception-specific types except never.

Cause there is no way to set error type in some cases like Promise, or exception throws, we can work with errors in rust-like style:

// Result<T, E> is the type used for returning and propagating errors.
// It is an sum type with the variants,
// Ok<T>, representing success and containing a value, and 
// Err<E>, representing error and containing an error value.
export type Ok<T> = { _tag: "Ok"; ok: T };
export type Err<E> = { _tag: "Err"; err: E };
export type Result<T, E> = Ok<T> | Err<E>;
export const Result = Object.freeze({
  Ok: <T, E>(ok: T): Result<T, E> => ({ _tag: "Ok", ok }),
  Err: <T, E>(err: E): Result<T, E> => ({ _tag: "Err", err }),
});

const start = (): Promise<Result<string, number>> => {
  return new Promise((resolve) => {
    resolve(someCondition ? Result.Ok("correct!") : Result.Err(-1));
  });
};

start().then((r) => {
  switch (r._tag) {
    case "Ok": {
      console.log(`Ok { ${r.ok} }`);
      break;
    }
    case "Err": {
      console.log(`Err { ${r.err} }`);
      break;
    }
  }
});

> The exception is typed any because we cannot guarantee the correct > type of the exception at design time, and neither TypeScript nor > JavaScript provide the ability to guard exception types at run time. Your best option is to use type guards to provide both a design-time and run-time check in your code.

source

What @EstusFlask mentioned in his answer is correct.

> But I want go one step near to an artificial solution to > simulate what we want with TypeScript capabilities.

 

Sometimes I use this pattern in my codes:

interface IMyEx{
   errorId:number;
}

class MyEx implements IMyEx{
   errorId:number;
   constructor(errorId:number) {
      this.errorId = errorId;
   }
}
// -------------------------------------------------------
var prom = new Promise(function(resolve, reject) {
     try {
         if(..........)
            resolve('Huuuraaa');         
         else
            reject(new MyEx(100));
	 }
     catch (error) {
            reject(new MyEx(101));
	 }
});

// -------------------------------------------------------
prom()
.then(success => {
	try {
		}
    catch (error) {
        throw new MyEx(102);
	}
})
.catch(reason=>{
    const myEx = reason as IMyEx;
	if (myEx && myEx.errorId) {
       console.log('known error', myEx)
    }else{
       console.log('unknown error', reason)
    }
})
	

You can use a proxy to explicitly force resolve and reject argument types. The following example does not try to mimic the constructor of a promise - because I didn't find that useful in practice. I actually wanted to be able to call .resolve(...) and .reject(...) as functions outside of the constructor. On the receiving side the naked promise is used - e.g., await p.promise.then(...).catch(...).

export type Promolve<ResT=void,RejT=Error> = {
  promise: Promise<ResT>;
  resolve: (value:ResT|PromiseLike<ResT>) => void;
  reject:(value:RejT) =>void
};

export function makePromolve<ResT=void,RejT=Error>(): Promolve<ResT,RejT> {
  let resolve: (value:ResT| PromiseLike<ResT>)=>void = (value:ResT| PromiseLike<ResT>)=>{}
  let reject: (value:RejT)=>void = (value:RejT)=>{}
  const promise = new Promise<ResT>((res,rej) => {
    resolve = res;
    reject = rej;
  });
  return { promise, resolve, reject };
}

The let statements look as if they are pointless - and they are pointless at runtime. But it stops compiler errors that were not easy to resolve otherwise.

(async()=>{
  const p = makePromolve<number>();
  //p.resolve("0") // compiler error
  p.resolve(0);
  // p.reject(1) // compiler error 
  p.reject(new Error('oops')); 

  // no attempt made to type the receiving end 
  // just use the named promise
  const r = await p.promise.catch(e=>e); 
})()

As shown, calls to .resolve and .reject are properly typed checked.

No attempt is made in the above to force type checking on the receiving side. I did poke around with that idea, adding on .then and .catch members, but then what should they return? If they return a Promise then it goes back to being a normal promise, so it is pointless. And it seems there is no choice but to do that. So the naked promise is used for await, .then and .catch.