What to log to debug Error [ERR_HTTP_HEADERS_SENT]? - node.js

Question

In an Express controller function, I am running into to this error Error [ERR_HTTP_HEADERS_SENT]. This occurs when I call res.json() if headers have already been set on the res object. However, I don't see place in my function (or middleware) where headers could be set prior to my calling res.json().
To debug the cause of this error, I thought I could add some logging. Prior to calling res.json, I could check if headers had been set and, if so, log some information about who set them.
async function get(req, res) {
...
if (res._header) {
logger.debug(...);
}
res.json(...);
Unfortunately, I don't see anything useful in the res object to log, any message that would indicate why/how the headers were set (or who set them). Any suggestions for what I could log to debug this issue? Or other debugging suggestions?

Answer 1

You can patch res.header res.send res.set to log the stack trace for you. For example this is my main application.
const express = require('express');
const app = express();
const someGoody = require('./stupid-blackbox');
/** patch the res **/
app.use((req, res, next) => {
const _header = res.header.bind(res); // .header and .set is an alias pair
const _send = res.send.bind(res);
res.header = res.set = (field, val) => {
console.trace('.header/.set called', field, val);
console.log('-----');
return _header(field, val);
}
res.send = (body) => {
console.trace('.send called', body);
console.log('-----');
return _send(body);
}
next();
})
// some innocent looking middleware
app.use(someGoody);
// my main routes
app.get('*', (req, res) => {
res.json({url: req.url});
})
app.listen(3000);
And for stupid-blackbox.js:
const express = require('express');
const router = express.Router();
router.use((req, res, next) => {
res.header('X-Crash-You', '1').send(':)');
next();
})
module.exports = router;
When ran, you will get this in the log:
Trace: .header/.set called X-Crash-You 1
at ServerResponse.res.header.res.set (C:\Users\eric_\Desktop\initial\play\index.js:11:13)
at router.use (C:\Users\eric_\Desktop\initial\play\stupid-blackbox.js:6:9)
at Layer.handle [as handle_request] (C:\Users\eric_\Desktop\initial\play\node_modules\express\lib\router\layer.js:95:5)
at trim_prefix (C:\Users\eric_\Desktop\initial\play\node_modules\express\lib\router\index.js:317:13)
at C:\Users\eric_\Desktop\initial\play\node_modules\express\lib\router\index.js:284:7
at Function.process_params (C:\Users\eric_\Desktop\initial\play\node_modules\express\lib\router\index.js:335:12)
at next (C:\Users\eric_\Desktop\initial\play\node_modules\express\lib\router\index.js:275:10)
at Function.handle (C:\Users\eric_\Desktop\initial\play\node_modules\express\lib\router\index.js:174:3)
at router (C:\Users\eric_\Desktop\initial\play\node_modules\express\lib\router\index.js:47:12)
at Layer.handle [as handle_request] (C:\Users\eric_\Desktop\initial\play\node_modules\express\lib\router\layer.js:95:5)
-----
Trace: .send called :)
at ServerResponse.res.send (C:\Users\eric_\Desktop\initial\play\index.js:17:13)
at router.use (C:\Users\eric_\Desktop\initial\play\stupid-blackbox.js:6:36)
at Layer.handle [as handle_request] (C:\Users\eric_\Desktop\initial\play\node_modules\express\lib\router\layer.js:95:5)
at trim_prefix (C:\Users\eric_\Desktop\initial\play\node_modules\express\lib\router\index.js:317:13)
at C:\Users\eric_\Desktop\initial\play\node_modules\express\lib\router\index.js:284:7
at Function.process_params (C:\Users\eric_\Desktop\initial\play\node_modules\express\lib\router\index.js:335:12)
at next (C:\Users\eric_\Desktop\initial\play\node_modules\express\lib\router\index.js:275:10)
at Function.handle (C:\Users\eric_\Desktop\initial\play\node_modules\express\lib\router\index.js:174:3)
at router (C:\Users\eric_\Desktop\initial\play\node_modules\express\lib\router\index.js:47:12)
at Layer.handle [as handle_request] (C:\Users\eric_\Desktop\initial\play\node_modules\express\lib\router\layer.js:95:5)
On the second line of each stack trace you can see the stupid-blackbox.js.
On a side note, res.json will not result in error if only res.header or res.set is called, the error is headers has been sent so that means somewhere in the code, res.send is called causing headers to be sent before your actual code.