The trouble with Express.js error handling (draft)

March 28, 2020

Async Code and Express.js

If a promise is rejected in Express, unless it’s handled explicitly, node will show an unhandled promise exception and the browser will hang, not showing any response from the server.

References

[1] Express error handling documentation https://expressjs.com/en/guide/error-handling.html

Errors that occur in synchronous code inside route handlers and middleware require no extra work. If synchronous code throws an error, then Express will catch and process it.
Shows the use of a package called method-override without saying anything about it in the text.
There is some noise in the examples, for example the use of methodOverride() which is irrelevant to the examople code.
It is not clear at all, but the custom error handler must come after the route handlers. This is discussed here https://stackoverflow.com/questions/29700005/express-4-middleware-error-handler-not-being-called

[2] Express 5 (in alpha since 2014) adds support for returns and rejected promises in the router. https://github.com/expressjs/express/blob/5.0/History.md

[3] Express 5 tracking issue https://github.com/expressjs/express/pull/2237

[4] Express 5 Promises https://github.com/expressjs/express/issues/2259

[5] Good article on how to improvess Express error handling https://nemethgergely.com/error-handling-express-async-await/

Shows how to return an error object with httpStatusCode.
Shows an example of how to wrap the request handler in an method to catch rejected promises.

[6] Article about “async middleware” https://dev.to/geoff/writing-asyncawait-middleware-in-express-6i0

[7] Good article about error handling in express https://thecodebarbarian.com/80-20-guide-to-express-error-handling

[8] Another article about error handling in express https://www.robinwieruch.de/node-express-error-handling

Error handling

Express requires that errors are passed to the next function, which is one of the inputs to the request handler.

app.post("/checkout", async (req, res, next) => {
  let responseData;
  try {
    responseData = await axios.get("https://example.com/give-me-an-error");
  } catch (e) {
    return next(e);
  }
}

And then you define an error handler:

// Error handler.
// eslint-disable-next-line no-unused-vars
app.use(function (err, req, res, next) {
  log.err(err.stack);
  res.status(500).send('Something went wrong, sorry.');
});

The problem

This is really verbose:

try {
  await webShopApi.finalizeOrder(order);
} catch (e) {
  return next(e);
}

This works, but doesn’t stop execution (so can only be used when execution ends anyway).

await webShopApi.finalizeOrder(order).catch(e);

Written by Jim Pickard who runs a pub by day and develops software by night. You can follow him on Twitter

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