Add PurgeCSS in production for Gatsby

gatsby-node.js

// Add PurgeCSS in production
// See: https://github.com/gatsbyjs/gatsby/issues/5778#issuecomment-402481270

const PurgeCssPlugin = require(`purgecss-webpack-plugin`)
const path = require(`path`)
const glob = require(`glob`)

const PATHS = {
  src: path.join(__dirname, `src`)
}

// Nonessential options removed for brevity
const purgeCssConfig = {
  paths: glob.sync(`${PATHS.src}/**/*.js`, { nodir: true }),
}

exports.onCreateWebpackConfig = ({ actions, stage }) => {
  if (stage.includes(`develop`)) return

  // Add PurgeCSS in production
  if (stage.includes(`build`)) {
    actions.setWebpackConfig({
      plugins: [new PurgeCssPlugin(purgeCssConfig)]
    })
  }
}