reproCSS

A CSS reprocessor using <style> tags

Can you imagine if you could interpolate JS inside CSS with the ${} syntax, and also control when and how frequently that CSS reprocessed with a process="" attribute on the <style> tag:

<style process="none"></style>
<style process="once"></style>
<style process="auto"></style>
<style process="touchstart mousedown"></style>

If you are using reproCSS with custom events, you may also optionally use a selector attribute specify a list of one or more CSS selectors you would like to add event listeners for. If no selector attribute is found all custom events will be applied to window.

<style process="click" selector="#any, .css, [selector]"></style>

You can add the CSS you want reprocss.js to apply to your HTML in <style> tags with the following values on the process attribute:

• none means no reprocessing
• once means process immediately and never again
• auto runs every resize, input, and click event on window
• any space-separated list of JS events you wish to listen for

How to use reproCSS

Github

To view reproCSS on Github visit: github.com/tomhodgins/reprocss

You can include the reprocss.js JavaScript plugin in your HTML:

<script src="reprocss.js"></script>

npm

If you are using reproCSS on NPM you can include it in your JS modules with a line like this:

const reprocss = require('reprocss')

How to write reproCSSed CSS

To evaluate JavaScript inside the CSS as it's being reprocessed by reprocss.js you can use the ${} interpolation syntax. The following <style> tag would always ensure the <div> in this example was half of the window's height:

<style process="auto">
  div {
    height: calc(${innerHeight}px / 2);
  }
</style>

When the browser is 1000px tall the ${innerHeight} in our CSS will be output as 500, leading to the following output:

<style process="auto">
  div {
    height: calc(500px / 2);
  }
</style>

Currently this plugin only supports <style> tags, but it may be possible to support CSS loaded via <link> with a similar technique.

Examples

Interpolating JS-supplied values in CSS content:;

<div>Hello</div>

<script>
  var user = 'Username'
</script>

<style process="once">
  div:after {
    content: ' ${user}';
  }
</style>

Element Queries via a JS Selector Resolver

<div id="demo">
  <p>Hello</p>
</div>

<style process="resize">
  ${demo.offsetWidth > 400 && "#demo"} {
    background: lime;
  }
  ${demo.offsetWidth > 400 && "#demo"} p {
    color: red;
  }
</style>

JS interpolation in CSS

<textarea id="demo"></textarea>

<style process="input">
  #demo {
    background: hsl(${demo.value.length}, 50%, 50%)
  }
</style>

Demos

• Element Queries with reproCSS