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npm-only way:
 run:
 npm install uglifyjs-folder --save-dev
and afterwards add to your package.json something like:
 "uglifyjs": "uglifyjs-folder js -eo jsm"
then run:
  npm run uglifyjs
Please note, if you'd need to generate to the same folder as the source files are (js), following should do the job:
 run:
 npm install del-cli uglifyjs-folder --save-dev
and afterwards add to your package.json something like:
 "uglifyjs": "del js/*.min.js; uglifyjs-folder js -eo js"
then run:
  npm run uglifyjs