Processors

Processors are hooks that transform generated CSS. Unlike transformers, which modify source code before extraction, processors run after UnoCSS has generated its CSS layers.

Define a processor

A processor receives the CSS for one layer and returns the CSS that should replace it. Both synchronous and asynchronous results are supported.

import type { CSSProcessor } from '@unocss/core'
import { defineConfig } from 'unocss'

const banner: CSSProcessor = {
  name: 'add-banner',
  order: 10,
  process(css, { layer, envMode }) {
    if (envMode !== 'build')
      return css
    return `/* generated layer: ${layer} */\n${css}`
  },
}

export default defineConfig({
  processors: [banner],
})

Processing flow

For every non-empty CSS layer, UnoCSS performs these steps:

1. Generate the raw layer CSS, including preflights and any enabled CSS layer wrapper or layer marker.
2. Sort processors by order in ascending order.
3. Pass the layer through each processor sequentially. The output of one processor becomes the input of the next.
4. Cache the processed layer and expose it through getLayer(), getLayers(), and css.

Different layers may be processed concurrently. A processor should avoid relying on mutable state shared between layers.

When setLayer() changes a layer, its callback receives the raw, unprocessed CSS. UnoCSS then runs the updated CSS through the complete processor chain again. This prevents processors from being applied repeatedly to their own previous output.

generated layer
  -> processor 1
  -> processor 2
  -> processed layer output

If a processor throws an error, generation fails and the error is passed to the caller.

Context

The second argument passed to process() is a CSSProcessorContext:

interface CSSProcessorContext<Theme extends object = object> {
  layer: string
  theme: Theme
  envMode: 'dev' | 'build'
}

• layer is the name of the current generated layer.
• theme is the resolved UnoCSS theme.
• envMode indicates whether UnoCSS is generating CSS for development or production builds.

Processor order

Processors with a lower order run first. Processors without an explicit order use 0.

processors: [
  { name: 'minify', order: 20, process: minify },
  { name: 'prefix', order: 10, process: addPrefixes },
]

In this example, prefix runs before minify.

Processors declared by presets and the user configuration are merged. The processor name identifies it when duplicate processors are removed.

Official processors

• Lightning CSS processor