PostCSS (Just In Time) Props

Only ship used variables! A CSS custom property helper based on PostCSS

postcss-jit-props watches for CSS variables and ensures a value entry exists in the stylesheet. Try in browser

This lets you provide a huge pool of properties for development and design, and rather than try and purge unused variables, only adds what was used.

CSS Before / During Development:

.foo {
  color: var(--brand-1);
  padding: var(--size-1) var(--size-2);
  margin-block-start: var(--size-2);
  animation: var(--fade-in);
}

@media (--dark) {
  .foo {
    color: white;
  }
}

CSS After / Result of Plugin:

+ @custom-media --dark (prefers-color-scheme: dark);

+ :root {
+   --brand-1: #81A1C1;
+   --size-1: 1rem;
+   --size-2: 2rem;
+   --fade-in: fade-in .5s ease;
+ }

.foo {
  color: var(--brand-1);
  padding: var(--size-1) var(--size-2);
  margin-block-start: var(--size-2);
  animation: var(--fade-in);
}

@media (--dark) {
  .foo {
    color: white;
  }
}

+ @keyframes fade-in {
+   to { opacity: 1; }
+ }

Step 1: Install plugin:

npm install --save-dev postcss-jit-props

Step 2: Add the plugin to plugins in postcss.config.js and pass it your props (CSS || JS || JSON).

Pass JS objects:

module.exports = {
  plugins: [
    require('postcss-jit-props')({
      '--brand-1': '#81A1C1',
      '--size-1': '1rem',
      '--size-2': '2rem',
      '--fade-in': 'fade-in .5s ease',
      '--fade-in-@': '@keyframes fade-in {to { opacity: 1 }}',
      '--dark': '@custom-media --dark (prefers-color-scheme: dark);',
      '--text': 'white',
      '--text-@media:dark': 'black',
    }),
    require('autoprefixer'),
  ]
}

or pass CSS files

module.exports = {
  plugins: [
    require('postcss-jit-props')({files: ['./props.css']}),
    require('autoprefixer'),
  ]
}

or JSON ✨

Javascript and JSON must use the -@ suffix on their custom property name in order for jit-props to find associated @keyframes

Configure where the selector the props are pushed to. Some CSS Module environments, for example, don't want the props in :root, so we can configure the plugin to push them where it's acceptable for the environment, like :global:

module.exports = {
  plugins: [
    require('postcss-jit-props')({
      ...MyProps,
      custom_selector: ':global'
    }),
    require('autoprefixer'),
  ]
}

Configure the @layer the props are pushed to. :

module.exports = {
  plugins: [
    require('postcss-jit-props')({
      ...MyProps,
      layer: 'design.system'
    }),
    require('autoprefixer'),
  ]
}