Express.js can't set headers after they are sent

Error: Can't set headers after they are sent.

NOTE If you are not using res.format() within the route function, this solution may not match your current issues

The error message is pretty clear, and the stack trace thrown with it gives you the exact line number of the error. But the code inducing it might not be so easy to make sense of. What’s wrong with the below code?

app.get('/foo', (req, res, next) => {
  return res.format({
    text: res.send('bar')
    html: res.send('<strong>bar</strong>')
  })
})

When the route function is executed (app.get()), the object literal within the res.format parenthesis are invoked before res.format itself. This means the response is in fact sent before res.format has even gotten started with setting the response headers. The correct way to go about this is to use functions as property values for the supplied object to res.format:

app.get('/foo', (req, res, next) => {
  return res.format({
    text: () => res.send('bar')
    html: () => res.send('<strong>bar</strong>')
  })
})