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Created March 1, 2021 10:27
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Getting a cloudflare worker to add resource hints to the head.
// set the site we are modifying
const site = '';
var resourceHints = `
<link rel="preload" href="/assets/font/font1.woff2" as="font" type="font/woff2" crossorigin="anonymous">
<link rel="dns-prefetch" href="">
<link href="" rel="preconnect" crossorigin>
// do this on a fetch
addEventListener('fetch', event => {
const request = event.request
async function handleRequest(request) {
// store the URL
const url = new URL(request.url);
// disallow crawlers (write a robots.txt file)
if(url.pathname === "/robots.txt") {
return new Response('User-agent: *\nDisallow: /', {status: 200});
// when overrideHost is used in a WPT script, WPT sets x-host to original host i.e. site we want to proxy
// store the value of x-host
const xhost = request.headers.get('x-host');
// If the `x-host` header is missing, abort and tell us
if(!xhost) {
return new Response('x-host header missing', {status: 403});
// set our hostname to that listed in the x-host header
url.hostname = xhost;
// look for header that allows us to bypass the transform entirely
const bypassTransform = request.headers.get('x-bypass-transform');
// get the accept header to allow us to examine the type of request it is
const acceptHeader = request.headers.get('accept');
// check that the x-host header matches what is contained in the site
// make sure we aren't wanting to bypass the transformations
if(xhost === site && (!bypassTransform || (bypassTransform && bypassTransform.indexOf('true') === -1))){
// check for an accept header and what it contains
if(acceptHeader && acceptHeader.indexOf('text/html') >= 0){
// store this particular HTML response for modification
let oldResponse = await fetch(url.toString(), request)
// create a new response
let newResponse = new HTMLRewriter()
// add listed resource hints to the page head
.on('head', new addResourceHints())
// transform the old response
// return the modified page
return newResponse
// otherwise just proxy the request unmodified
return fetch(url.toString(), request);
class addResourceHints {
element(element) {
// notice how we are prepending the hints, right after the opening head tag
// can be changed to append if you want them right before the closing tag
element.prepend(resourceHints, {html: true});
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