Use of old ES code

[TheLukaszNs]

In the index.js file we're using old var syntax (const and let would be a better choice) and normal anonymous functions, instead of shorter and better arrow functions:

• Now:

  router.use(function (req, res, next) { next() })

• What we could use:

  router.use((req, res, next) => next())

Those changes won't affect the app at all, but the code would become more readable.