Throw await syntax in Nodejs async express handler is not working

Question

const express = require("express");
const expressAsyncHandler = require("express-async-handler");

const app = express();

const f = async () => {
  return false;
};

app.get(
  "/",
  expressAsyncHandler(async () => {
    throw await f();
  }),
  () => {
    console.log("the bug!");
  }
);

app.use((err, req, res, next) => {
  console.log("caught!", err);
});

app.listen(4000, () => console.log("listening on port 4000..."));

Expected output on the console:

"caught!".

output:

the bug!.

question: Why? Is it a bug in async-express-handler package or is it a normal JavaScript behaviour? what if I want to throw await something inside? how ?

This is the entire soruce code: https://github.com/Abazhenov/express-async-handler/blob/master/index.js The error is just caught so nothing is passed to your error handler. — Konrad, Aug 29 '22 at 22:21
@KonradLinkowski, important, I updated the code, I caught another bug — Normal, Aug 29 '22 at 22:22
I don't see a bug here. Why wouldn't the next middleware run when the error is handled already? — Konrad, Aug 29 '22 at 22:24
Error is handled by the `catch` so it's not unhandled anymore and code can proceed — Konrad, Aug 29 '22 at 22:26
It's called in the library `Promise.resolve(fnReturn).catch(next)` — Konrad, Aug 29 '22 at 22:28
What you're not getting is the standard behavior is for a Promise to call the `reject` method when the `resolve` method throws an exception. There's nothing to see here. Move along. — Tibrogargan, Aug 29 '22 at 22:37
We will know when maintainers will answer on [your issue](https://github.com/Abazhenov/express-async-handler/issues/55) — Konrad, Aug 29 '22 at 22:37

Bergi · Accepted Answer · 2022-08-30T00:58:42.050

Your problem is that you're throwing the value false. This doesn't fit into nodejs' callback conventions (and by extension, express error handling), which requires the err parameter to get a truthy value to be considered an error. A much simpler way to reproduce the issue:

app.get(
  "/",
  (req, res, next) => {
    Promise.reject(false).catch(next);
    // or even just:
    next(false);
  },
  () => {
    console.log("the bug!");
  }
);

So just don't do that! Always throw an Error, not a string, not something else, and this principle also holds for promise rejections.

Throw await syntax in Nodejs async express handler is not working

1 Answers1