Why doesn't typescript complain when I DON'T return a Promise in an async function

Issue

I have an async function which I intend to return an object of type MyResponseType. As I understand, an async function must return a Promise, so I created the following:

import mongoose from 'mongoose'
import MyModel from './models/MyModel'
import { MyResponseType } from "../common/types";

const myFunc = async (
  title: string,
  text?: string
): Promise<MyResponseType> => {
  const result = await MyModel.create({
    title,
    text,
  });
  return {
    message: 'Success',
    result
  };
};

My questions are:

1. Is this the right way to return from an async function, assuming I want the result in a resolved state when the caller calls await myFunc({...})?

2. Why doesn't typescript complain that the return type is MyResponseType, even though it is expecting Promise<MyResponseType>? Is there some implicit conversion going on?

Solution

1. Is this the right way to return from an async function, assuming I want the result in a resolved state when the caller calls await myFunc({...})?

Yes. (But it's a fulfilled state, not [just] a resolved state. More about promise terminology in my blog post.)

2. Why doesn't typescript complain that the return type is MyResponseType, even though it is expecting Promise<MyResponseType>? Is there some implicit conversion going on?

Not conversion, but yes, something implicit. async functions are syntactic sugar for promise creation and consumption. They always return promises. The value you provide with return ___ is the fulfillment value used to fulfill the implicit promise they create.

In your example, myFunc runs synchronously until after it calls MyModel.create; once it has the promise from create, it passes it to await and the function creates and returns its own promise. Later, asynchronously, when the promise from create is settled, one of two things happens:

• If the promise from create is fulfilled, the logic of myFunc continues and, in this case, fulfills the promise from myFunc with the object you provide to return.

• If the promise from create is rejected, your function's promise is rejected with the rejection reason the create promise provided.

(That's slightly simplified, but the gory details aren't necessary here.)

Answered By - T.J. Crowder