Awesome Open Source
Awesome Open Source


Create offline-ready web apps with service workers, social meta, async assets and cache-busting.

Clojars Project CircleCI

So it's like a layer of knowledge about the real world on top of Hiccup.


Out of the box:

  • Meta for SEO, Twitter, Facebook (Open Graph), link sharing
  • Precaching Service Worker generation based on Workbox
  • Clojure stylesheets with garden
  • Clojure markup rendered with hiccup
  • Built-in cache-busting for assets
  • Async stylesheets loading
  • And also: that responsive viewport=something meta tag and language tag


Java 8 or later.


1. Define a page

(ns pages.home)

(defn page [req]
   ; essentials
  {:title "Lightpad"
   :body [ [:h1 "Ah, a Page!"]]
   :head-tags [[:meta {:name "custom" :property "stuff"}]]
   :stylesheet-async "large-stuff.css" ; injects an async renderer(s)
   :script "/app.js" ; async by default
   :garden-css [:h1 {:font-size :20px}] ; critical path css

   ; seo and meta
   :description "Like a notepad but cyberpunk"
   :og-image ""
   :twitter-site "@lightpad_ai"

   ; PWA stuff
   :manifest    true
   :lang        "en"
   :theme-color "hsl(0, 0%, 96%)"
   :service-worker "/service-worker.js" ; will inject also a service worker lifecycle script
   :sw-default-url "/app"
   :sw-add-assets ["/icons/fonts/icomoon.woff", "/lightning-150.png"]})

2. Wire it up to your routes (e.g. Compojure)

(ns server
 (:require [page-renderer.api :as pr]
           [compojure.core :refer [defroutes GET]] 
           [pages.home :as p]))

  (GET "/" req
   {:status 200
    :headers {"Content-Type" "text/html"}
    :body (pr/render-page (p/page req)})

  (GET "/service-worker.js" req
   {:status 200
    :headers {"Content-Type" "text/javascript"}
    ; will generate a simple Workbox-based service worker on the fly with cache-busting
    :body (pr/generate-service-worker (p/page req))})

  (GET "/quicker-way" req (pr/respond-page (p/page req))))

3. Celebrate

Page output
<!DOCTYPE html>
    <meta charset="utf-8">
    <link href="/favicon.png" rel="icon" type="image/png">
    <meta content="width=device-width, initial-scale=1, maximum-scale=1" name="viewport">
    <meta content="Some bird stuff" name="description">
    <meta content="summary" name="twitter:card">
    <meta content="" name="twitter:site">
    <meta content="Some bird stuff" name="twitter:description">
    <meta content="" name="twitter:image">
    <meta content="Page" property="og:title">
    <meta content="Some bird stuff" property="og:description">
    <meta content="" property="og:image">
    <style id="inline-css--garden">
    h1 {
    font-size: 20px;
    <!-- Service Worker Lifecycle Snippet -->
    import { Workbox } from '';
    const promptStr = 'New version of the application is downloaded, do you want to update? May take two reloads.';
    function createUIPrompt(opts) {
      if (confirm(promptStr)) {
    if ('serviceWorker' in navigator) {
      const wb = new Workbox('/service-worker.js');
      wb.addEventListener('waiting', (event) => {
        const prompt = createUIPrompt({
          onAccept: async () => {
            wb.addEventListener('activated', (event) => {
              console.log('sw-init: activated')
            wb.addEventListener('controlling', (event) => {
              console.log('sw-init: controlling')
            wb.messageSW({type: 'SKIP_WAITING'});
<body class="page">
    <h1>Ah, a Page!</h1>
    var link = document.createElement('link');
Service Worker

    { url: '/heavy-stuff.css', revision: 'file-hash' },
    { url: '/fonts/icomoon.woff', revision: 'file-hash' },
    { url: '/lightpad/compiled/app.js', revision: 'file-hash' },
    { url: '/favicon.png', revision: 'file-hash' },
    { url: '/app', revision: 'file-hash' }
], { ignoreURLParametersMatching: [/hash/] })

    workbox.precaching.getCacheKeyForURL('/app'), {
        whitelist: [ /^\/app/ ],
        blacklist: [ /^\/app\/service-worker.js/ ]

workbox.routing.setCatchHandler(({event}) => {
    console.log('swm: event ', event)

addEventListener('message', (event) => {
    if ( && === 'SKIP_WAITING') {
        console.log('swm: skipping waiting')

self.addEventListener('activate', () => {
    console.log('swm: activated')

self.addEventListener('install', () => {
    console.log('swm: installed')


Use page-renderer.api namespace.

(defn ^String render-page [renderable])

Produces an html string.

(defn ^String generate-service-worker [renderable])

Produces a JavaScript ServiceWorker script text. Service worker will additionally load Workbox script.

(defn ^Map respond-page [renderable])

Produces Ring compatible response map with status 200.

(defn ^Map respond-service-worker [^Map renderable])

Produces Ring compatible response map with status 200.

renderable – is a map that may have the following fields

  • ^Vector :body - a vector for Hiccup to render into HTML of the document's body
  • ^String :title - content for title tag
  • ^String :favicon - favicon's url
  • ^String :script - script name, will be loaded asynchronously
  • ^String :stylesheet - stylesheet filename, will be plugged into the head, will cause browser waiting for download.
  • ^String :stylesheet-async - stylesheet filename, will be loaded asynchronously by script.
  • ^String :garden-css - data structure for Garden CSS
  • ^String :script-sync - script name, will be loaded synchronously
  • ^String :js-module - entry point for JS modules. If you prefer your scripts to be served as modules
  • ^Boolean :skip-cachebusting? - will skip automatic cachebusting if set. Defaults to false.
  • ^String :on-dom-interactive-js - a js snippet to run once DOM is interactive or ready.
  • ^String/Collection<String> :stylesheet-inline - stylesheet filename, will be inlined into the head.
PWA related
  • ^String :link-image-src - url to image-src

  • ^String :link-apple-icon - url to image used for apple-touch-icon link

  • ^String :link-apple-startup-image - url to image used for apple-touch-startup-image link

  • ^String :theme-color - theme color for PWA (defaults to white)

  • ^String/Boolean :manifest - truthy value will add a manifest link If a string is passed – it'll be treated as a manifest url. Otherwise '/manifest.json' will be specified.

  • ^String/Boolean :service-worker - service worker url, defaults to /service-worker.js

  • ^String :sw-default-url – application default url. Must be an absolute path like '/app'. Defaults to '/'. Will be used in a regexp.

  • ^List<String> :sw-add-assets - a collection of additional assets you want to precache, like ["/fonts/icon-font.woff" "/logo.png"]

More meta
  • ^String :lang - when provided will render a meta tag and a document attribute for page language.
  • ^String :meta-title - content for the title tag (preferred)
  • ^String :meta-keywords - content for the keywords tag
  • ^String :meta-description - meta description
  • ^Map :meta-props – meta which must be rendered as props {'fb:app_id' 123}
  • ^String :head-tags - data structure to render into HTML of the document's head
Open Graph meta (@link
  • ^String :og-title - OpenGraph title
  • ^String :og-description - OpenGraph description
  • ^String :og-image - absolute url to image for OpenGraph
  • ^String :og-type - OpenGraph object type
  • ^String :og-url - OpenGraph page permalink
Twitter meta (@link
  • ^String :twitter-site - Twitter @username. Required for all Twitter meta to render
  • ^String :twitter-creator - Twitter @username.
  • ^Keyword :twitter-card-type - Twitter card type one of #{:summary :summary_large_image :app :player}
  • ^String :twitter-description - Twitter card description
  • ^String :twitter-image - Twitter image link. Twitter images are useu
  • ^String :twitter-image-alt - Twitter image alt

Service Worker generation

page-renderer allows you to produce a full-blown offline-ready PWA fast. Your users will be able to "install" it as a PWA app on mobile platforms or as Chrome app on desktop platforms. All you need to do is just add another route to your scheme.

How it works

If you use service-worker field then page-renderer will generate a precaching service worker. The worker utilizes Workbox (by Google) and will precache all the assets that you've defined in renderable, and will be able to serve them offline. It also does proper cache-busting with hashes. page-renderer will also inject a service worker lifecycle management script into your page so that your users will be prompted to download a newer version of your website when it's ready.

How cache-busting works here

page-renderer provides very basic, but bulletproof cache-busting by providing a url param with content-hash (or last modification timestamp), like /file?hash=abec112221122. For every stylesheet, script and image on resource paths – it will generate a content hash. If the file can't be found on the classpath or inside a local resources/public directory it will receive the library load time, roughly equaling the application start time.

Where to see in action:

Personally I use it for all my website projects including:


If you are using a frontend proxy server like Nginx – don't forget to prevent it from serving service-worker as a static asset. My js assets block looks like this

        location ~ ^(?!/service-worker).*\.(?:js|css|svg)$ {
                etag off;
                expires 1y;
                gzip_vary on;
                add_header Cache-Control "public";
                access_log off;

Also note the switched off etag. If you use page-renderer you can turn off etag and use expires header only for more aggressive caching and preventing avoidable requests. See details in this thread on StackOverflow.


Copyright © 2019 Ivan Fedorov

Distributed under the MIT License.

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