next.js/packages/next/src/build/webpack/plugins/css-minimizer-plugin.ts

import cssnanoSimple from 'next/dist/compiled/cssnano-simple'
import postcssScss from 'next/dist/compiled/postcss-scss'
import postcss from 'postcss'
import type { Parser } from 'postcss'
import { webpack, sources } from 'next/dist/compiled/webpack/webpack'
import { getCompilationSpan } from '../utils'

// https://github.com/NMFR/optimize-css-assets-webpack-plugin/blob/0a410a9bf28c7b0e81a3470a13748e68ca2f50aa/src/index.js#L20
const CSS_REGEX = /\.css(\?.*)?$/i

type CssMinimizerPluginOptions = {
  postcssOptions: {
    map: false | { prev?: string | false; inline: boolean; annotation: boolean }
  }
}

export class CssMinimizerPlugin {
  __next_css_remove = true

  private options: CssMinimizerPluginOptions

  constructor(options: CssMinimizerPluginOptions) {
    this.options = options
  }

  optimizeAsset(file: string, asset: any) {
    const postcssOptions = {
      ...this.options.postcssOptions,
      to: file,
      from: file,

      // We don't actually add this parser to support Sass. It can also be used
      // for inline comment support. See the README:
      // https://github.com/postcss/postcss-scss/blob/master/README.md#2-inline-comments-for-postcss
      parser: postcssScss as any as Parser,
    }

    let input: string
    if (postcssOptions.map && asset.sourceAndMap) {
      const { source, map } = asset.sourceAndMap()
      input = source
      postcssOptions.map.prev = map ? map : false
    } else {
      input = asset.source()
    }

    return postcss([cssnanoSimple({ colormin: false }, postcss)])
      .process(input, postcssOptions)
      .then((res) => {
        if (res.map) {
          return new sources.SourceMapSource(res.css, file, res.map.toJSON())
        } else {
          return new sources.RawSource(res.css)
        }
      })
  }

  apply(compiler: webpack.Compiler) {
    compiler.hooks.compilation.tap('CssMinimizerPlugin', (compilation: any) => {
      const cache = compilation.getCache('CssMinimizerPlugin')
      compilation.hooks.processAssets.tapPromise(
        {
          name: 'CssMinimizerPlugin',
          stage: webpack.Compilation.PROCESS_ASSETS_STAGE_OPTIMIZE_SIZE,
        },
        async (assets: any) => {
          const compilationSpan =
            getCompilationSpan(compilation) || getCompilationSpan(compiler)
          const cssMinimizerSpan = compilationSpan!.traceChild(
            'css-minimizer-plugin'
          )

          return cssMinimizerSpan.traceAsyncFn(async () => {
            const files = Object.keys(assets)
            await Promise.all(
              files
                .filter((file) => CSS_REGEX.test(file))
                .map(async (file) => {
                  const assetSpan = cssMinimizerSpan.traceChild('minify-css')
                  assetSpan.setAttribute('file', file)

                  return assetSpan.traceAsyncFn(async () => {
                    const assetSource = compilation.getAsset(file).source
                    const etag = cache.getLazyHashedEtag(assetSource)
                    const cachedResult = await cache.getPromise(file, etag)

                    assetSpan.setAttribute(
                      'cache',
                      cachedResult ? 'HIT' : 'MISS'
                    )
                    if (cachedResult) {
                      compilation.updateAsset(file, cachedResult)
                      return
                    }

                    const result = await this.optimizeAsset(file, assetSource)
                    await cache.storePromise(file, etag, result)
                    compilation.updateAsset(file, result)
                  })
                })
            )
          })
        }
      )
    })
  }
}