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.

Example

CSS Before / During Development:
``css
.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:
`diff
+ @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; }
+ }
`

Usage

Step 1: Install plugin:

`sh
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:
`js
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

`js
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:

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

Configure the @layer the props are pushed to. :

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