404 because of restarting the webpack-dev-server
Instead of using
entry: [ './app/index.js'],as your entry
add two additional entries along with it like this:
entry: [ 'webpack-dev-server/client?http://localhost:3000', // WebpackDevServer host and port 'webpack/hot/only-dev-server', // "only" prevents reload on syntax errors './app/index.js' // Your appʼs entry point]