index.js

addEventListener("fetch", event => {
  event.respondWith(handleRequest(event))
})


const BUCKET_NAME = "[YOUR_BUCKET_NAME]"
const BUCKET_URL = `https://storage.googleapis.com/${BUCKET_NAME}`

async function serveAsset(event) {
  const url = new URL(event.request.url)
  const cache = caches.default
  let response = await cache.match(event.request)
  
  if (`${BUCKET_URL}${url.pathname}` != `${BUCKET_URL}/`) {
    if (!response) {
      response = await fetch(`${BUCKET_URL}${url.pathname}`)
      const headers = { "cache-control": "public, max-age=14400", 'Access-Control-Allow-Origin': '*' }
      response = new Response(response.body, { ...response, headers })
      event.waitUntil(cache.put(event.request, response.clone()))
    }
    return response
  } else {
    return new Response("Method not allowed", { status: 405 })
  }
  
}

async function handleRequest(event) {
  if (event.request.method === "GET") {
    
    let response = await serveAsset(event)
    if (response.status > 399) {
      response = new Response(response.statusText, { status: response.status })
    }
    return response
  } else {
    return new Response("Method not allowed", { status: 405 })
  }
}