require.extensions is not supported by webpack. Use a loader instead
The best solution for this is using webpack-node-externals to not worry about node_modules at all when working with webpack for backend.
When bundling with Webpack for the backend - you usually don't want to bundle its node_modules dependencies. This library creates an externals function that ignores node_modules when bundling in Webpack.
I had the same problem and I solved it by adding the following code to my webpack.config.js file:
resolve: { alias: { 'express-handlebars': 'handlebars/dist/handlebars.js' }}
This set up uses ejs and the ejs-specific solution is to set this in your webpack config:
resolve: { alias: { 'ejs': 'ejs.min.js' }}