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Make a CF worker modify a CSS file and / or a JavaScript file on the fly.
// 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()
* Make your HTML changes here
// return the modified page along with custom headers
return newResponse
} else if(acceptHeader && acceptHeader.indexOf('text/css') >= 0){// Change CSS here
// grab the CSS response
const response = await fetch(url.toString(), request);
// extract the body of the request
let body = await response.text();
// modify the CSS response body
body = body.replace(/Arial, Helvetica Neue, Helvetica, sans-serif;/gi,'Georgia, Times, Times New Roman, serif;').replace(/Arial,Helvetica Neue,Helvetica,sans-serif;/gi,'Georgia, Times, Times New Roman, serif;')
// return the modified response
return new Response(body, {
headers: response.headers
} else if(acceptHeader && acceptHeader.indexOf('*/*') >= 0){// Change JavaScript here (uses the generic Accept directive)
// being granular we only modify a single response for a specific JavaScript file
// grab the JS response
const response = await fetch(url.toString(), request);
// extract the JS body of the request
var body = await response.text();
// using template literals we add a console.log to the end
body = `${body} console.log('String added last');`;
// return the modified response
return new Response(body, {
headers: response.headers
// otherwise just proxy the request unmodified
return fetch(url.toString(), request)
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