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"