Install & run:
npm i webpack@2.1.0-beta.20
./node_modules/.bin/webpack
Output with UglifyJsPlugin
Asset Size Chunks Chunk Names
index.min.js 2.59 kB 0 [emitted] main
Output without UglifyJsPlugin
Asset Size Chunks Chunk Names
index.min.js 2.59 kB 0 [emitted] main
index.min.js.map 2.54 kB 0 [emitted] main
UglifyJsPlugin
is preventing generation of sourcemaps in Webpack 2