How to log an Expressjs response body

Both Express Request and Response are NodeJs Streams. With this in mind An Express Request is readable NodeJS Stream, which means that it provide a data event that a developer can listen for in order to see the data/payload being received from the client.

The Express Response on the other hand, is implemented as a writable NodeJS Stream. And for some reasons, writable streams do not provide a a data event. Therefore, there is no straightforward way of capturing the data being written (ex: if you want to log them).

After checking how Express js implements the send method, i came up with a naive way of capturing the response payload.

Steps

• Create a middleware

app.use((req, res, next) => {
  // ... response payload capture logic
})
// your api handler here

• override the res.end function inside the middleware

app.use((req, res, next) => {
    res.end = function(chunk) {
        console.log('[res.end] chunk', typeof chunk, chunk)
        end.apply(res, arguments)
      }
    next()
})

NOTES

• This piece of code was tested with express@4.16.2
• This piece of code works if you use res.send or res.json
• This piece of code does NOT works if you use res.sendFile (in this case you will need to do more work )