SHELL

rm is not recognized as internal command

rm is only meant to be used in UNIX systems. If you see that issue on Windows,
then you either need to run your command through Git Bash 
or just install the following npm library:

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npm install --save-dev rimraf mkdirp
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Make sure you then replace the UNIX parts like the examples bellow:

before:
"build": "rm -rf ./build && mkdir ./build && babel -d ./build ./src"

after:
"build": "rimraf ./build && mkdirp ./build && babel -d ./build ./src"