How to annotate generator functions in typescript

Question

Let's look at the example async function from promisejs.org , which is allows us to wait for promises using generators:

function async(makeGenerator){
  return function () {
    var generator = makeGenerator.apply(this, arguments);

    function handle(result){
      // result => { done: [Boolean], value: [Object] }
      if (result.done) return Promise.resolve(result.value);

      return Promise.resolve(result.value).then(function (res){
        return handle(generator.next(res));
      }, function (err){
        return handle(generator.throw(err));
      });
    }

    try {
      return handle(generator.next());
    } catch (ex) {
      return Promise.reject(ex);
    }
  }
}

Example usage:

var login = async(function* (username, password, session) {
  var user = yield getUser(username);
  var hash = yield crypto.hashAsync(password + user.salt);
  if (user.hash !== hash) {
    throw new Error('Incorrect password');
  }
  session.setUser(user);
});

My question: How should both of these functions be annotated in TypeScript, in orded to keep type safety?

What I've tried: I know the async function returns a Promise<T> , but I'm not sure what T should be. I guess it should depend on the generator passed in, but what type does a generator have? There is GeneratorFunction in the typings for nodejs or bluebird (can't remember which), but that one is not generic so I can't do async(makeGenerator: GeneratorFunction<T>): Promise<T> as I'd want to.

Answer 1

but I'm not sure what T should be

In your case login returns nothing to Promise<void> .