How to throw exception from async function

Question

I have an async function that I expect to throw exception on failure. However something seems to preventing this:

by omitting the try catch blocks I expect an exception to be thrown which I want to handle outside of the function.

The actual result I get is somewhat confusing:

(node:10636) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): E11000 duplicate key error index.

(node:10636) DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

async f(obj) {     await db.collection('...').save(obj); } 

I get the same result when I try to catch the exception and throw something else instead:

async f(obj) {     try {         await db.collection('...').save(obj);     } catch(e) {         throw e.message;     } } 

The function is called from a try block, so don't see how this is an Unhandled Promise.

I am trying to use f as a parameter of another function:

g(obj, handler) {     try {         handler(obj);     } catch(e);         ...     } }  g(objToSave, f); 

Answer 1

async f(obj) {     try {         await db.collection('...').save(obj);     } catch(e) {         throw e.message;     } } 

The function is called from a try block, so don't see how this is an Unhandled Promise.

What is unhandled here is the rejection of a promise returned by the f() function, not by the .save() method. So this will not cause that problem:

async f(obj) {     try {         await db.collection('...').save(obj);     } catch(e) {         console.error(e.message);     } } 

Throwing an exception in async function always rejects the promise that is returned by that function.

To catch the exception you either have to do this in another async function:

try {     asyncFunc(); } catch (err) {     // you have the error here } 

or you can add a rejection handler explicitly:

asyncFunc().catch(err => {     // you have the error here }); 

If you are catching the exception and throwing another exception then you get the same problem, just in a different function.

You either have to add a promise rejection handler and not throw an exception or return a rejected promise there - or run that function in another async function that handles the exception instead of rethrowing the same or new exception.

To sum it up: Every async function returns a promise. Every promise needs to have a rejection handler.

The rejection handler is added using a two-function .then() or with .catch(), or with try { await asyncFunction(); } catch (err) { ... }

When you have a promise rejection with no rejection handler, you will get a warning in older versions of Node and a fatal error in newer versions of Node - see this answer for more details:

• Should I refrain from handling Promise rejection asynchronously?