How to reject in async/await syntax?

A better way to write the async function would be by returning a pending Promise from the start and then handling both rejections and resolutions within the callback of the promise, rather than just spitting out a rejected promise on error. Example:

async foo(id: string): Promise<A> {
    return new Promise(function(resolve, reject) {
        // execute some code here
        if (success) { // let's say this is a boolean value from line above
            return resolve(success);
        } else {
            return reject(error); // this can be anything, preferably an Error object to catch the stacktrace from this function
        }
    });
}

Then you just chain methods on the returned promise:

async function bar () {
    try {
        var result = await foo("someID")
        // use the result here
    } catch (error) {
        // handle error here
    }
}

bar()

Source - this tutorial:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise

Your best bet is to throw an Error wrapping the value, which results in a rejected promise with an Error wrapping the value:

} catch (error) {
    throw new Error(400);
}

You can also just throw the value, but then there's no stack trace information:

} catch (error) {
    throw 400;
}

Alternately, return a rejected promise with an Error wrapping the value, but it's not idiomatic:

} catch (error) {
    return Promise.reject(new Error(400));
}

(Or just return Promise.reject(400);, but again, then there's no context information.)

In your case, as you're using TypeScript and foo's return value is Promise<A>, you'd use this:

return Promise.reject<A>(400 /*or Error*/ );

In an async/await situation, that last is probably a bit of a semantic mis-match, but it does work.

If you throw an Error, that plays well with anything consuming your foo's result with await syntax:

try {
    await foo();
} catch (error) {
    // Here, `error` would be an `Error` (with stack trace, etc.).
    // Whereas if you used `throw 400`, it would just be `400`.
}

You can create a wrapper function that takes in a promise and returns an array with data if no error and the error if there was an error.

function safePromise(promise) {
  return promise.then(data => [ data ]).catch(error => [ null, error ]);
}

Use it like this in ES7 and in an async function:

async function checkItem() {
  const [ item, error ] = await safePromise(getItem(id));
  if (error) { return null; } // handle error and return
  return item; // no error so safe to use item
}

It should probably also be mentioned that you can simply chain a catch() function after the call of your async operation because under the hood still a promise is returned.

await foo().catch(error => console.log(error));

This way you can avoid the try/catch syntax if you do not like it.