Running an handler after response is returned

[danbars]

Hi,

In some cases you'd want to run common code after one of the handlers returned a Response but before it was returned to the client. Some examples:

• Async send accumulated log-lines to a logger service such as logDNA
• Async send events to an analytics service
• Enrich the response with additional headers such as requestId, context, cookies, etc

For all these cases I'd like to be able to add handlers that will run as middleware on the way out.

What do you think? Is there another pattern to achieve that?

[kwhitley]

Hey @danbars!

Absolutely this is a pattern that is used, for example to embed CORS heads into responses before sending. The way to achieve this is simply chain onto the router.handle promise:

// pseudocode

const logResponse = response => log(response) // assumes "log()" returns the response as well

router
  .handle(request, env) // assuming you return a valid Response from this...
  .then(response => {
    // do something to response

    return response // and pass it along
  })
  .then(logResponse) // basically tacking on response middleware
  .then(response => {
    // do something else to response

    return response // and pass it along
  })

Anecdotally (and unrelated), there are no rules within itty to enforce what may be passed through (Responses are just one option), so you could use this for all sorts of weird things that I haven't thought of.

Hope this helps!

[danbars]

Awesome, thanks 👍