The bug

res.render rendered with a callback that threw the error the template engine reported:

app.render(file, data, options, (err, html) => {
  if (err) throw err
  res.send(html)
})

That works when the engine calls back synchronously, since the throw unwinds into the handler and applyHandler catches it. Many engines call back asynchronously though, e.g. after reading the template file, and then the error is thrown outside of any handler. It surfaced as an uncaught exception, which terminates the process by default, and the request never got a response.

app.engine('eta', (_path, _data, _options, cb) => setImmediate(() => cb(new Error('template failed'))))
app.get('/', (_req, res) => res.render('index.eta'))

The fix

res.render passes the error to next(), so it reaches the app's error handler like any other error. This matches express, whose default res.render callback calls req.next(err).

renderTemplate takes next as a new optional last argument, extendMiddleware passes it. Without it the error is still thrown, so direct callers of renderTemplate behave as before.

Tests

tests/core/app.test.ts, new res.render suite:

• an error of an engine calling back asynchronously reaches the error handler (an uncaught exception before)
• an error of an engine calling back synchronously reaches the error handler