Add an Example using event.waitUntil() for caching a request

[GregBrimble]

It let's you respond rapidly, and then save data, which can be used to speed up subsequent requests.

https://developers.cloudflare.com/workers/about/tips/fetch-event-lifecycle/

let url = 'https://ca88e957.ngrok.io/log'

addEventListener('fetch', event => {
  event.respondWith(handleRequest(event))
  event.waitUntil(postLog(JSON.stringify({hello: 'hi'})))
})

function postLog(data) {
  return fetch(url, {
    method: 'POST',
    body: data,
  })
}

/**
 * Respond to the request
 * @param {Request} request
 */
async function handleRequest(event) {
  url = 'https://ca88e957.ngrok.io/magic'
  return new Response('hello world', {status: 200})
}

https://ca88e957.ngrok.io/magic is fetched after the repsonse is sent back.