Nuxt/lib/builder/webpack/postcss.js

const { existsSync } = require('fs')
const { resolve } = require('path')
const debug = require('debug')('nuxt:postcss')
const { isPureObject } = require('../../common/utils')
const { cloneDeep } = require('lodash')

module.exports = function postcssConfig() {
  let config = cloneDeep(this.options.build.postcss)

  if (!config) {
    return false
  }

  // Search for postCSS config file and use it if exists
  // https://github.com/michael-ciniawsky/postcss-load-config
  for (let dir of [this.options.srcDir, this.options.rootDir]) {
    for (let file of ['postcss.config.js', '.postcssrc.js', '.postcssrc', '.postcssrc.json', '.postcssrc.yaml']) {
      if (existsSync(resolve(dir, file))) {
        const postcssConfigPath = resolve(dir, file)
        debug(`Using config file: ${postcssConfigPath}`)
        return {
          sourceMap: this.options.build.cssSourceMap,
          config: {
            path: postcssConfigPath
          }
        }
      }
    }
  }

  // Normalize
  if (Array.isArray(config)) {
    config = { plugins: config }
  }

  // Apply default plugins
  if (isPureObject(config)) {
    config = Object.assign({
      useConfigFile: false,
      sourceMap: this.options.build.cssSourceMap,
      plugins: {
        // https://github.com/postcss/postcss-import
        'postcss-import': {
          root: this.options.rootDir,
          path: [
            this.options.srcDir,
            this.options.rootDir,
            ...this.options.modulesDir
          ]
        },

        // https://github.com/postcss/postcss-url
        'postcss-url': {},

        // http://cssnext.io/postcss
        'postcss-cssnext': {}
      }
    }, config)
  }

  // Map postcss plugins into instances on object mode once
  if (isPureObject(config) && isPureObject(config.plugins)) {
    config.plugins = Object.keys(config.plugins)
      .map(p => {
        const plugin = require(this.nuxt.resolvePath(p))
        const opts = config.plugins[p]
        if (opts === false) return // Disabled
        const instance = plugin(opts)
        return instance
      }).filter(e => e)
  }

  return config
}