Referance : (https://designmodo.com/bootstrap-4-npm-grunt-sass/) , (https://designmodo.com/bootstrap-4-npm-grunt-sass-2/)
For compile .scss files into one css grunt sass
We can also ask Grunt to watch for changes and run the tasks it needs. The watch task will run every time a file is changed and saved. Since the configuration was already set up on our Sass files we just need to simply type grunt watch on command prompt or terminal.