Log request that caused error in node modules - node.js

Question

I run a NodeJS server with two new error types in the logs:
[2021-05-21T09:11:33.891Z] SyntaxError: Unexpected token h in JSON at position 0
at JSON.parse (<anonymous>)
at createStrictSyntaxError (~/server/node_modules/body-parser/lib/types/json.js:158:10)
at parse (~/server/node_modules/body-parser/lib/types/json.js:83:15)
at ~/server/node_modules/body-parser/lib/read.js:121:18
at invokeCallback (~/server/node_modules/raw-body/index.js:224:16)
at done (~/server/node_modules/raw-body/index.js:213:7)
at IncomingMessage.onEnd (~/server/node_modules/raw-body/index.js:273:7)
at IncomingMessage.emit (events.js:323:22)
at IncomingMessage.EventEmitter.emit (domain.js:482:12)
at endReadableNT (_stream_readable.js:1204:12)
at processTicksAndRejections (internal/process/task_queues.js:84:21)
The stacktrace shows only node_modules paths, not where in my code this error may have started. The stdout logs do not show what could have originated this error around that time either.
The server code that handles JSON objects is:
// Use JSON parser (required to parse POST forms)
app.use((req, res, next) => {
bodyParser.json()(req, res, next);
});
app.use(bodyParser.urlencoded({extended: true}));
I added logging inside this function in case I have the same error in the future.
In general, how can I log information about the request that caused an error in the node modules?
update with client-side code
This error originated from a user and I am unable to replicate it. The client-side code sending JSON data is:
// `id` indicates the ID of the video
var body = {
percent: percent,
videoId: id,
eventLabel: eventLabel
}
async function view() {
return await fetch("/viewership", {
method: "POST",
credentials: "include",
headers: {
Accept: "application/json",
"Content-Type": "application/json"
},
body: JSON.stringify(body)
});
};
The Network tab of the Chrome debugger shows this request payload:
{percent: 0, videoId: ..., eventLabel: "play"}

Answer 1

Well, here's what we know.
The code is processing an IncomingMessage (and incoming http request)
The error comes from the body-parser module
The error comes from JSON.parse() on what is apparently supposed to be a JSON body
The error appears to come from this particular section of code in the body-parser module.
That code is this:
if (strict) {
var first = firstchar(body)
if (first !== '{' && first !== '[') {
debug('strict violation')
throw createStrictSyntaxError(body, first)
}
}
So, it is apparently failing to find a leading { or [ on the JSON and is instead finding an h.
We can deduce from that information that an incoming http request (probably a POST) is supposed to have a JSON body, but the data is not legal JSON.
Your first point of debugging is to see exactly the JSON body data is in the request. If this request is coming from a browser, you can look in the Chrome network tab of the debugger and see exactly what the browser is sending your server.
So, this is most likely a client-caused error. Either the content-type is set wrongly to JSON when the data is not JSON or the client is supposed to be sending JSON, but is not sending proper JSON.
If you can show us the client-side code for this, we may be able to spot the error in that code.
Do you know a way to log any request that throws an error, e.g. for other bad requests in the future unrelated to JSON?
When the body-parser gets bad JSON, it calls the Express error handler with the exception. If you go the the "Writing Error Handlers" on this Express doc page, it will show you how to catch these errors and handle them with some error page back to the client and as much logging as you want.