How to reject in async/await syntax?

How can I reject a promise that returned by an async/await function?

e.g. Originally:
foo(id: string): Promise {
return new Promise((resolve, reject) => {
someAsyncPromise().then((value)=>resolve(200)).catch((err)=>reject(400))
});
}


Translate into async/await:
async foo(id: string): Promise {
try{
await someAsyncPromise();
return 200;
} catch(error) {//here goes if someAsyncPromise() rejected}
return 400; //this will result in a resolved promise.
});
}


So, how could I properly reject this promise in this case?

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, you'd use this:

return Promise.reject(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`.
}

Stack Overflow Q#42453683, score: 524