fix linter errors

[caffeinum]

#2 #3 #1

fixed compiling errors and linter warnings.

I used Node 10.14 if that changes anything.

I recommend nvm (https://github.com/creationix/nvm#install-script) and Node no lower than 8 version

git clone https://github.com/coffeemipt/simple-pos-in-node.git

# install and run backend
cd server
npm i
npm run start &

# install and run frontend
cd ../react-pos
npm i
npm run start

[caffeinum]

@DenieceChan @chemscobra @sumitsaha are you interested in the project yet?