React is a very nice front-end framework, javascript is nice, but what if you need an android app? Or an iOS app for that matter? You have all read of apps that embed a browser, and in the browser javascript runtime, you'd load a full application, written in react or angular or what have you. Maybe pure javascript. 

keytool -genkey -v -keystore ~/.android/debug.keystore \
  -alias debug_keystore -keyalg RSA -keysize 2048 -validity 10000

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