[Webpack] Externalize Dependencies to be Loaded via CDN with webpack

We can separate our custom application code from the common libraries we leverage, such as React and ReactDOM. In this lesson we'll configure webpack to treat React and ReactDOM as external dependencies. Then we'll update our HTML template to conditionally include the CDN links for those libraries for production builds.

Why to do this?

One of the main advantages of using a CDN is reduced latency, so if you cannot, for some reason, host your main bundle on a CDN, you can at least speed up loading of some dependencies. In addition to that, if you use a commonly used CDN, you can potentially take advantage of the browser's cached copy of React and ReactDOM from other sites that are also hosting from that CDN

The reduced latency comes from the fact that a CDN will serve files from a POP (point of presence) that is geographically closer to the requester. The reduced latency is a pure physics problem of distance. This doesn't solve other network issues like congestion and throughput limits, but it's a step in the right direction.

We only want to do this for production:

// webpack.config.prod.js

const merge = require('webpack-merge')
const {BundleAnalyzerPlugin} = require('webpack-bundle-analyzer')
const baseConfig = require('./webpack.config.base')

module.exports = merge(baseConfig, {
  mode: 'production',
  plugins: [new BundleAnalyzerPlugin({
    analyzerMode: 'static',
    openAnalyzer: false,
    reportFilename: 'bundle_sizes.html'
  })],
  externals: {
    react: 'React',
    'react-dom': 'ReactDOM'
  }
})

We add 'exernals' prop, it is a key value pair, key is the package name, value is the variable name:

import React from 'react'
import ReactDOM from 'react-dom'

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