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Created March 2, 2021 00:30
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Using a CF Worker to add `async` or `defer` to a script tag in the page
// set the site we are modifying
const site = '';
// do this on a fetch
addEventListener('fetch', event => {
const request = event.request
const url = new URL(request.url)
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 this header is missing, abort
if(!xhost) {
return new Response('x-host header missing', {status: 403});
// set our hostname to that listed in the xhost 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');
if(xhost === site && (!bypassTransform || (bypassTransform && bypassTransform.indexOf('true') === -1))){
if(acceptHeader && acceptHeader.indexOf('text/html') >= 0){
// store this particular request for modification
let oldResponse = await fetch(url.toString(), request)
// create a new response
let newResponse = new HTMLRewriter()
// add defer to this script
.on("script[src*='script-to-be-deferred.js']", new addDeferAttribute())
// add async to this script
.on("script[src*='script-to-be-asynced.js']", new addAsyncAttribute())
// transform the page
// return the modified page along with custom headers
return newResponse
// otherwise just proxy the request unmodified
return fetch(url.toString(), request)
class addDeferAttribute {
element(element) {
element.setAttribute('defer', 'defer');
class addAsyncAttribute {
element(element) {
element.setAttribute('async', 'async');
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