javascript - react - what are service workers in chrome

“Status Code:200 OK(from ServiceWorker)” in Chrome Network DevTools? (2)

I am familiar with http status codes, but recently I saw a strange line in my chrome debugger. Instead of ordinary Status Code:200 OK I saw the following: Status Code:200 OK (from ServiceWorker) .

My guess is that this just tells me that ServiceWorker is somehow responsible for accessing this document, but this is just random guess. Can anyone authoritatively (without guesses, with links to respected resources) tell me what does this mean and what are the implications?

This is a common source of confusion, so I wanted to go into a bit more detail.

I have a full working demo in this Gist , and you can view a live version of it thanks to RawGit.

Here's the relevant portion of the service worker code inline, for illustrative purposes:

self.addEventListener('fetch', event => {
  if (event.request.url.endsWith('one.js')) {
    // Requests for one.js will result in the SW firing off a fetch() request,
    // which will be reflected in the DevTools Network panel.
  } else if (event.request.url.endsWith('two.js')) {
    // Requests for two.js will result in the SW constructing a new Response object,
    // so there won't be an additional network request in the DevTools Network panel.
    event.respondWith(new Response('// no-op'));

  // Requests for anything else won't trigger event.respondWith(), so there won't be
  // any SW interaction reflected in the DevTools Network panel.

And here's what a filtered version of the Network panel looks like in Chrome 48 when that service worker is in control of a page, and requests are made for one.js , two.js , and three.js :

Our service worker's fetch handler will do one of three things:

  • If it's a request for one.js , it will fire off a fetch() request for the same URL, and then call event.respondWith() using that response. The first one.js entry in the screenshot, the one with "(from ServiceWorker)" in the "Size" column, is there by virtue of the fact that we called event.respondWith() inside the fetch handler. The second one.js entry in the screenshot, the one with the little gear icon next to it and "(from cache)" in the "Size" column, represents that fetch() request that was made inside the service worker while responding to the event. Since the actual one.js file was already in the HTTP cache at the point I took this screenshot, you see "(from cache)", but if the HTTP cache didn't have that response already, you would see an actual network request along with the response size.
  • If it's a request for two.js , it will handle the request by constructing a new Response object "from thin air". (Yes, you can do that!) In this case, we are calling event.respondWith() , so there's an entry for two.js with "(from ServiceWorker)" in the "Size" column. But unlike with one.js , we're not using fetch() to populate the response, so there's no additional entry in the Network panel for two.js .
  • Finally, if it's a request for three.js , our service worker's fetch event handler won't actually call event.respondWith() . From the perspective of the page, and also from the perspective of the Network panel, there's no service worker involvement with that request, which is why there's just a "(from cache)" rather than "(from ServiceWorker)" in the "Size" column.