You should be using uglify-es.

[trusktr]

Using uglify-js is likely to leave your users of some modern browsers with zero-configuration errors.

F.e. you can find many bugs like this one:

meteor/meteor#9224

Just use uglify-es

[100terres]

Webpack also use it: https://webpack.js.org/plugins/uglifyjs-webpack-plugin/

[davidnagli]

If we would be using uglify-es, we would solve #8

We need to implement this ASAP