CDN with B2 + CF Workers

'use strict';

const b2Url = 'https://f002.backblazeb2.com';
const b2Bucket = 'BUCKET_NAME';

const b2UrlPath = `/file/${b2Bucket}/`;
addEventListener('fetch', event => {
    return event.respondWith(fileReq(event));
});

// define the file extensions we wish to add basic access control headers to
const corsFileTypes = ['png', 'jpg', 'gif', 'jpeg', 'webp'];

// backblaze returns some additional headers that are useful for debugging, but unnecessary in production. We can remove these to save some size
const removeHeaders = [
    'x-bz-content-sha1',
    'x-bz-file-id',
    'x-bz-file-name',
    'x-bz-info-src_last_modified_millis',
    'X-Bz-Upload-Timestamp',
    'x-bz-info-user',
    'Expires'
];
const expiration = 31536000; // override browser cache for images - 1 year

// define a function we can re-use to fix headers
const fixHeaders = function(url, status, headers){
    let newHdrs = new Headers(headers);
    // add basic cors headers for images
    if (corsFileTypes.includes(url.pathname.split('.').pop())) {
        newHdrs.set('Access-Control-Allow-Origin', '*');
    }
    // override browser cache for files when 200
    if (status === 200) {
        newHdrs.set('Cache-Control', "public, max-age=" + expiration);
    } else {
        // only cache other things for 5 minutes
        newHdrs.set('Cache-Control', 'public, max-age=300');
    }
    // set ETag for efficient caching where possible
    const ETag = newHdrs.get('x-bz-content-sha1') || newHdrs.get('x-bz-info-src_last_modified_millis') || newHdrs.get('x-bz-file-id');
    if (ETag) {
        newHdrs.set('ETag', ETag);
    }
    // remove unnecessary headers
    removeHeaders.forEach(header => {
        newHdrs.delete(header);
    });
    return newHdrs;
};

async function fileReq(event) {
    const cache = caches.default; // Cloudflare edge caching
    const url = new URL(event.request.url);

    let response = await cache.match(url); // try to find match for this request in the edge cache

    if (response) {
        // use cache found on Cloudflare edge. Set X-Worker-Cache header for helpful debug
        let newHdrs = fixHeaders(url, response.status, response.headers);
        newHdrs.set('X-Worker-Cache', "true");
        return new Response(response.body, {
            status: response.status,
            statusText: response.statusText,
            headers: newHdrs
        });
    }

    // no cache, fetch image, apply Cloudflare lossless compression
    response = await fetch(b2Url + '/file/' + b2Bucket + url.pathname,
        {cf: {polish: "lossless"}}
    );
    let newHdrs = fixHeaders(url, response.status, response.headers);
    response = new Response(response.body, {
        status: response.status,
        statusText: response.statusText,
        headers: newHdrs
    });

    event.waitUntil(cache.put(url, response.clone()));
    return response;
}