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Complexities of using CSS in modern web apps and how ClojureScript can leverage Garden to generate style sheets at runtime

#clojurescript #css #web


We attempt replacing all style management magic provided by webpack with about 60 lines of ClojureScript and, arguably, achieve astounding success.

Some people think that CSS is about building cascades of styles. It is not, fortunately. Sadly, newcomers to web development tend to confuse cascading style inheritance, which is a good thing, with a necessity to build said cascades themselves using oh so helpful tools such as LESS or SASS. This leads to a unmaintainable mess of code which you’ve probably encountered multiple times in different projects.

.book {
  &_container {
    font-size: 14px;
    color: #333;

    .title {
      font-size: 16px;
      font-weight: bold;

      &:hover {
        color: #666;

    &_placeholder {
      color: #eee;
      text-transform: uppercase;

      &_line {
        white-space: nowrap;
        margin-bottom: 1em;

        &_letter {
          margin-right: 10px

          &:last-child {
            margin-right: 0;

      &--empty {
        background-color: #f00;

Well, you get a general idea. Somehow people are trying to:
a) manually do work that a browser is supposed to do, having a CSS engine and all that;
b) delay having to think about architecture by using complex macro-like magic.

You can add to this mess Javascript libraries like Modernizr1, or the ones that sniff device type2, it’s dpi, orientation, and screen size and add corresponding CSS classes to the body3 so you can target your styles more precisely.

Confronted with such level of complexity, the best course of action is usually to throw it all away, stop and rethink your life priorities let computer do as much of your job as possible. And this is exactly what libraries like webpack’s css-loader4 do currently. You just require your CSS files as you would your regular *.js files and receive a map containing all classes from imported style sheet.

// fancy-button.jsx

require React from 'react';
require cs from 'class-names';

require styles from 'button.css';

export default class FancyButton extends React.Component {
  render() {
    const {priority, children} = this.props;

    const priorityStyle = styles[priority] || styles.primary;
    const className = cs(styles.button, priorityStyle);

    return (
      <button className={className}>{children}</button>

That button.css file might look like snippet below. Note, that because css-loader prefixes all classes, there is a need to denote cases where prefixing should be omitted. For this, following syntax is used: :global(.class).

// button.css

.button {
  border-radius: 3px;
  border: none;

.primary {
  background-color: blue;
  color: red;

.error {
  background-color: red;
  color: green;

.button :global(.fa-icon) {
  display: none;

Everything is magically included, CSS is parsed, all classes camel cased for your convenience and, more importantly, automagically prefixed to include file’s name in class names (i.e. .button gets turned into .fancy-button__button based on name of file wherein CSS file was included, or whatever prefixing settings you specify). This ensures class names won’t collide and you had to do pleasantly little to enjoy it.

But this creates another problem: of managing large number of CSS files. While before we had everything taken care of by diligent SASS compiler, now we are forced to rely on a plugin inside webpack. And while access to CSS classes in JS is a great improvement, we still can’t manipulate our styles through code in a consistent way. Things like CSS variables5 promise us some flexibility, but given the time it takes to implement such controversial features in all major browsers, we shouldn’t rely on them in foreseeable future. CSS and JS still live in completely different and largely isolated worlds – without DOM acting as a glue there is no easy way of accessing styles that are to be applied.

In a project I’m currently working on, we have a rather complicated way of building CSS bundle. It involves separately compiling SASS files, then running through webpack dependency graph identifying and transforming required CSS files, running autoprefixer an all those files and finally concatenating everything and minifying result.

While this is obviously capable of producing production-ready results, I don’t think one would call it straightforward. Too much magic is happening inside of webpack, too many moving parts and all sorts of configuration is needed to make this work in a desirable way. Moreover, there is regrettably no control over, or indication of, order parts of CSS are concatenated in. More than once this forced me to resort to !important in unobvious places.

Meet Clojure(Script)

The running theme in Clojure world is to simplify everything until things are so simple it would be very difficult to simplify them any further.

When you look at React wrappers, for instance, you’ll notice that, with a notable exception of OM6, they mostly focus on using pure functions for rendering, simple atoms for state and some form of array-based HTML description language7.

Continuing this tradition, we’ll use Garden8 to manage our stylesheets. In order to achieve feature parity with our previous setup we need to implement following:

  1. Generated stylesheets with prefixed classes;
  2. Inclusion of appropriate styles;
  3. A convenient way to generate prefixed classnames.

To have something to start from, lets take a simple Icon component and use it as a baseline.

(ns hoarder.components.icon
  (:require [rum.core :as rum]
            [hoarder.css.manager :refer [include-styles pref]]))

(def styles
    {:position "relative"
     :display "inline-block"
     :overflow "hidden"
     :line-height "100%"
     :fill "currentColor"
    {:height "50px"
     :width "50px"
     :font-size "50px"}]
    {:height "25px"
     :width "25px"
     :font-size "25px"}]
    {:width "100%"
     :height "100%"
     :background "inherit"
     :fill "inherit"
     :pointer-events "none"
     :transform "translateX(0)"
     :-ms-transform "translate(.5px, -.3px)"}]])

;; Add comments

(rum/defc icon < (include-styles styles :icon) ;; RUM mixin
  [iname iclass]
  (let [c (pref :icon)] ;; class name generator
    [:div {:class [(c (or iclass "default")) (c "icon")] :key iname}
     [:svg {:class (c "container")}
      [:use {:xlink-href (str "/img/icons/" iname ".svg#icon")}]]]))

Here we define some simple styles, create a RUM component, then include styles, and use prefixed class names in component’s template.

Now all we need is to define a minimal implementation of our style sheet management thing so that our component will at least render.

Stylesheet generation

Garden can do many things, including, but not limited to, unit calculations, color manipulation and, obviously, it can generate CSS.

While I’m not entirely sure using hiccup-like syntax for CSS is a good thing, we’ll leave this discussion for later time. For now, let’s agree this is a manageable way to represent complex AST of a modern CSS preprocessor and roll with it.

(defn prefix-with
  "Prefix all class names or ids in a chain
  (prefix-with :test \"\") #=> \"#test-id.test-hello.test-world\""
  [prefix subj]
   (clojure.string/replace subj #"([\.#])(.+?)" (str "$1" (stringify-keyword prefix) "-" "$2"))))

(defn type-match? [t v] (= t (type v)))

(defn prefix-styles
  "Recursively prefix Garden classes and ids"
  [prefix form]
  (condp type-match? form
    cljs.core/PersistentVector (mapv (partial prefix-styles prefix) form)
    cljs.core/Keyword (prefix-with prefix (name form))
    js/String (prefix-with prefix form)

(defn make-style'
  "Create renderable styles"
  [prefix st]
  (css (prefix-styles prefix st)))

(def make-style (memoize make-style'))

After some testing, it appears that all modern browsers are fast enough at concatenating strings. Except Firefox. This little bastard managed to add almost 100ms every time styles were rendered. Thus, because memory is cheap and I sure didn’t want to spend time investigating why Firefox is the only one behaving this way, memoizing9 style generation function was chosen as a passable remedy for those annoying delays.

DOM manipulation

Now that we have a string with CSS, we need a way to load it into a browser. Also, we need a way to rename class names in such a way, that would ensure we bind nicely to CSS classes we generated previously.

(defn insert-styles
  "Inserts Stylesheet into document head"
  [styles id]
  (let [el (.createElement js/document "style")
        node (.createTextNode js/document styles)]
    (.setAttribute el "id" id)
    (.appendChild el node)
    (.appendChild (.-head js/document) el)

(defn stringify-keyword [kw]
  (-> kw str (subs 1) (clojure.string/replace "/" "-")))

(defn rum-prefix
  "Prefix RUM classes"
  [p cls]
  (let [prefix (stringify-keyword p)
        mkpref #(str prefix "-" %)]
    (if (vector? cls)
      (map mkpref cls)
      (mkpref cls))))

(defn pref
  "Helper function for easy class name prefixing"
  (partial rum-prefix prefix))

(defn render-stylesheet [style prefix]
  (let [prefixed-styles (make-style prefix style)
        id (stringify-keyword prefix)]
    (if-let [el (.getElementById js/document id)]
       (aset el "innerHTML" prefixed-styles)
       (comment .appendChild el (.createTextNode js/document prefixed-styles)))
      (insert-styles prefixed-styles id))))

(defn include-styles
  "RUM mixin to automagically load prefixed styles"
  [styles prefix]
  (let [f (fn [state]
            (render-stylesheet styles prefix)
    {:will-mount f}))

Here, everything is pretty straightforward. Styles get inserted in a style with a specific id, or, if such id already exists, styles get updated with new ones.

A RUM mixin tries to update styles when component tries to mount itself. Obvious added benefit of this approach is naturally hierarchical order of <style/> elements in <head/>.

It is reasonable to assume that using (ns-name *ns*) as a fallback prefix value would simplify default use case, but I haven’t tried it yet.

This constitutes a minimal system needed to get reloadable, modifiable and in-situ style sheets for your components. While depending on a rather large libraries to generate CSS and to reload code on the fly10, the code to manage styles is extremely small and concise. There is absolutely no reason why such approach couldn’t be implemented in pure Javascript, and, arguably, this has already been adopted in React Native11.

Bundling CSS inside JS payload is nothing new12 and is obviously not of everyone. Benefit of reducing the number of moving parts, reducing number of cached/requested files and radically simplifying manifest.json my be desirable for B2B apps, electron13 wrappers, webapps14 and generally less initial-load-time-sensitive apps, is rather an impediment if you develop highly latency-optimized or content-heavy sites. But then you probably wouldn’t be using Clojurescript in the first place, I think.

(Un)intended consequences

While being able to manipulate units15 and macro-like media-queries and keyframes16 in Garden are a nice thing to have, in my experience have a somewhat limited use.

What I found a lot more powerful is ability to seamlessly integrate Clojurescript with stylesheets. Use functions to generate code, access DOM and/or JS APIs whenever needed, generate CSS without having to put up with whatever syntax SASS decided was OK to have for it’s mixins.

(def mobile-padding
  "iOS status bar offset"
  (if standalone? {:padding-top "20px"}))

(defn random-background-url []
  (str "url(" "/img/backgrounds/" (Math/floor (rand 6)) ".jpg)"))

;; Mostly iOS 10 colors
(def colors
  {:red "255, 59, 48"
   :orange "255, 149, 0"
   :yellow "255, 204, 0"
   :green "76, 217, 100"
   :teal-blue "90, 200, 250"
   :blue "0, 122, 255"
   :purple "88, 86, 214"
   :pink "255, 45, 85"
   :black "30, 30, 31" ; #1e1e1f
   :white "247, 247, 247"
   :true-white "255, 255, 255"
   :text "3, 3, 3"
   :text-soft "146, 146, 146"
   :shadow "22, 28, 34"
   :bluish "228, 233, 234" ; #e4e9ea

(defn palette-color
  "Genereate a color from palette with a given opacity"
  ([color-name] (palette-color color-name 1.0))
  ([color-name opacity]
   (if-let [color (get colors color-name)]
     (str "rgba(" color ", " opacity ")")
     (palette-color :black))))

;; Spinner animation using keyframes

(defkeyframes sk-stretchdelay
  [["0%, 40%, 100%" {:transform "scaleY(0.4)"}]
   ["20%" {:transform "scaleY(1.0)"}]])

(def styles
    {:width "50px"
     :height "40px"
     :text-align "center"
     :font-size "10px"}
     {:background-color (palette-color :green)
      :height "100%"
      :width "6px"
      :margin-right "2px"
      :display "inline-block"
      :-webkit-animation "sk-stretchdelay 1.2s infinite ease-in-out"
      :animation "sk-stretchdelay 1.2s infinite ease-in-out"}]
    [:.rect2 {:animation-delay "-1.1s"}]
    [:.rect3 {:animation-delay "-1.0s"}]
    [:.rect4 {:animation-delay "-0.9s"}]
    [:.rect5 {:animation-delay "-0.8s"}]]])

As you’ve probably guessed by now, one can easily compose styles by requiring needed pieces and adding them as a node whenever needed. Keeping libraries of often-used styles will enable not only easy reuse, but efficient tree-shaking of unused branches, provided for free by Clojurescirpt compiler.


It is pretty obvious that approach described above isn’t even remotely ideal. It’s not intended to be. The idea was to reduce perceived complexity and make underlying style manipulations predictable and manageable. Optimizing for a larger set of constraints is left as an exercise for the reader.

  1. “Modernizr” ↩︎

  2. “device.js” ↩︎

  3. “restive.js” ↩︎

  4. “css-loader” ↩︎

  5. “CSS variables” ↩︎

  6. “OM cljs” ↩︎

  7. “Hiccup” ↩︎

  8. “Garden” ↩︎

  9. “clojure.core/memoize” ↩︎

  10. “adzerk-oss/boot-reload” ↩︎

  11. “Styles in React Native” ↩︎

  12. “Do We Even Need CSS Anymore?” ↩︎

  13. “Electronjs” ↩︎

  14. “Web app manifest” ↩︎

  15. “garden units” ↩︎

  16. “garden keyframes” ↩︎