async function calculating hash of static folders and adding to DefinePlugin with webpack.merge

webpack.js

const fs = require('fs');
const webpack = require('webpack');
const merge = require('webpack-merge');
const TerserPlugin = require('terser-webpack-plugin');
const config = require('config');
const { hashElement } = require('folder-hash');
const base = require('./webpack.base');
const packageJson = require('./package.json');

const definePluginConfig = {
  VERSION: JSON.stringify(packageJson.version),
  DEBUG: config.debug,
  MOCK_CONTENT: false,
};

if (config.mock) {
  definePluginConfig.MOCK_CONTENT = JSON.stringify(fs.readFileSync('./mock.txt').toString().split('\n'));
}

module.exports = async () => {
  const hash = await hashElement('build', {
    folders: {
      exclude: ['info', 'fonts'],
    },
    files: {
      exclude: ['*bundle.*.js', '*.html'],
    },
  });

  const hashes = {};
  for (let i = 0, len = hash.children.length; i < len; i++) {
    const folder = hash.children[i];
    hashes[`${folder.name.toUpperCase()}_HASH`] = JSON.stringify(folder.hash);
  }

  base.plugins.shift();

  return merge(base, {
    mode: 'production',
    output: {
      filename: 'bundle.[contenthash].min.js',
    },
    devtool: false,
    performance: {
      maxEntrypointSize: 900000,
      maxAssetSize: 900000,
    },
    optimization: {
      minimizer: [
        new TerserPlugin({
          terserOptions: {
            output: {
              comments: false,
            },
          },
        }),
      ],
    },
    plugins: [
      new webpack.DefinePlugin(Object.assign({}, definePluginConfig, hashes)),
    ],
  });
};