Adaptive Image Resize

In this document, we demonstrate how to identify the client type via User-Agent in the request header, use fetch API to get the image from the origin, and convert it to the format required by the browser.
addEventListener('fetch', event => {
// If the function code throws an unhandled exception, Edge Functions forwards the current request back to the origin.

async function handleEvent(event) {
const { request } = event;
const urlInfo = new URL(request.url);
const userAgent = request.headers.get('user-agent');

// Request a non-image resource
if (!/\.(jpe?g|png)$/.test(urlInfo.pathname)) {
return fetch(request);

// Image width on the mobile device
let width = 480;
const isPcClient = isPc(userAgent);

// Image width on the PC
if (isPcClient) {
width = 1280;

// Scale the image
const response = await fetch(request, {
eo: {
image: {

// Set the response header
response.headers.set('x-ef-client', isPcClient ? 'pc' : 'mobile');
return response;

// Identify the request client type
function isPc(userAgent) {
const regex = /(phone|pad|pod|iPhone|iPod|ios|iPad|Android|Mobile|BlackBerry|IEMobile|MQQBrowser|JUC|Fennec|wOSBrowser|BrowserNG|WebOS|Symbian|Windows Phone)/i;
if(regex.test(userAgent)) {
return false;
return true;

Sample Preview

In the address bar of the browser, enter a URL (such as https://example.com/images-resize/ef-1.jpeg) that matches a trigger rule of the edge function to preview the effect of the sample code.
For displaying on PC, scale the image to 1280 x 720.

For displaying on mobile devices, scale the image to 480 x 270.