Just open index.html and it will work :)
If you know git, checkout to the branch 'es6' for a cleaner ES6 version (but could not work in all browsers)
Nice input from @Takumari85: https://codepen.io/Takumari85/pen/RaYwpJ
Nice checkbox from @hansmaad: https://codepen.io/hansmaad/pen/qaGrQL