Understanding x => x * 2 and other modern javascript

For anyone who does not understand line 2 below, there is a nice explanation (regarding modern javascript) at:

https://gist.github.com/gaearon/683e676101005de0add59e8bb345340c

and

https://developer.mozilla.org/en-US/docs/Web/JavaScript

let x := 30;
x => x * 2;
alert(x)

Place the above as button code in Ninox to test it.