400/500 error handler in Express - sending JSON vs HTML - html

Question

We have this error handler in Express:
app.use(function (err, req, res, next) {
res.status(err.status || 500);
const stck = String(err.stack || err).split('\n').filter(function (s) {
return !String(s).match(/\/node_modules\// && String(s).match(/\//));
});
const joined = stck.join('\n');
console.error(joined);
const isProd = process.env.NODE_ENV === 'production';
const message = res.locals.message = (err.message || err);
const shortStackTrace = res.locals.shortStackTrace = isProd ? '' : joined;
const fullStackTrace = res.locals.fullStackTrace = isProd ? '': (err.stack || err);
if (req.headers['Content-Type'] === 'application/json') {
res.json({
message: message,
shortStackTrace: shortStackTrace,
fullStackTrace: fullStackTrace
});
}
else {
//locals for template have already been set
res.render('error');
}
});
My question is - we want to send back either JSON or HTML depending on the type of request. I assume looking at Content-Type header is the best way to do this. Is there any other way I should be checking?
Isn't the Content-Type header sometimes called 'content-type' (lowercase)?

Answer 1

I prefer using the following (this is for a 404 error but that's not important):
if (req.accepts('html')) {
// Respond with html page.
fs.readFile('404.html', 'utf-8', function(err, page) {
res.writeHead(404, { 'Content-Type': 'text/html' });
res.write(page);
res.end();
});
} else {
if (req.accepts('json')) {
// Respond with json.
res.status(404).send({ error: 'Not found' });
} else {
// Default to plain-text. send()
res.status(404).type('txt').send('Not found');
}
}
Basically you can use the req object's accepts method to find out what you should send as a response.