vue-cli/packages/@vue/cli-plugin-babel/index.js

module.exports = (api, options) => {
  const useThreads = process.env.NODE_ENV === 'production' && options.parallel
  const cliServicePath = require('path').dirname(require.resolve('@vue/cli-service'))

  api.chainWebpack(webpackConfig => {
    const jsRule = webpackConfig.module
      .rule('js')
        .test(/\.jsx?$/)
        .exclude
          .add(filepath => {
            // always trasnpile js in vue files
            if (/\.vue\.jsx?$/.test(filepath)) {
              return false
            }
            // exclude dynamic entries from cli-service
            if (filepath.startsWith(cliServicePath)) {
              return true
            }
            // check if this is something the user explicitly wants to transpile
            if (options.transpileDependencies.some(dep => filepath.match(dep))) {
              return false
            }
            // Don't transpile node_modules
            return /node_modules/.test(filepath)
          })
          .end()
        .use('cache-loader')
          .loader('cache-loader')
          .options(api.genCacheConfig('babel-loader', {
            '@babel/core': require('@babel/core/package.json').version,
            '@vue/babel-preset-app': require('@vue/babel-preset-app').version,
            'babel-loader': require('babel-loader/package.json').version
          }, 'babel.config.js'))
          .end()

    if (useThreads) {
      jsRule
        .use('thread-loader')
          .loader('thread-loader')
    }

    jsRule
      .use('babel-loader')
        .loader('babel-loader')
  })
}