Currently, errors that happen in the framework express layer are passed to the default express error handler. The default error handler returns the error to the client with a stack trace if in non-prod enviornments. It would be useful for certain cases for the user to have context of what happened during a request instead of the user being bypassed entirely. Instead, if the user has an error handler middleware installed in their application, we should pass the framework error to the user's error handler.

For example, if the framework gets a request with a bad json body.

const app = express():

app.post("/", (req, res) => {
 ...
});

// User error handler
app.use((err, req, res, next) => {
  logger.log(err);
  res.send("Caught error!");
});

functions.http("function, app);

// Post request with bad JSON
http.post("/", "{"id": "Hello}");

The framework responds with the following not very helpful message from the default Express error handler:

SyntaxError: Expected double-quoted property name in JSON at position 20 (line 3 column 1)
    at JSON.parse (<anonymous>)
    at parse (functions-framework-nodejs/node_modules/body-parser/lib/types/json.js:92:19)
    at functions-framework-nodejs/node_modules/body-parser/lib/read.js:128:18
    at AsyncResource.runInAsyncScope (node:async_hooks:211:14)
    at invokeCallback (functions-framework-nodejs/node_modules/raw-body/index.js:238:16)
    at done (functions-framework-nodejs/node_modules/raw-body/index.js:227:7)
    at IncomingMessage.onEnd (functions-framework-nodejs/node_modules/raw-body/index.js:287:7)
    at IncomingMessage.emit (node:events:518:28)
    at endReadableNT (node:internal/streams/readable:1698:12)
    at process.processTicksAndRejections (node:internal/process/task_queues:90:21)

This change introduces a propagate error to client middleware that when enabled, propagates errors to the user client application error middleware chain, where we give power to the user to handle the error, log details, or do whatever they want to do:

// Post request with bad JSON
http.post("/", "{"id": "Hello}");

The framework responds with a user defined message from the user's error handler!:

Don't forget the user also logged this error, giving them more visibility and control of their application. I think this is a useful change for users who want a bit more autonomy.