Change all variable declaration to ES6

[creesch]

So no more var and instead let and const:

const for variables that will not be updated.
let for variables that (possibly) will be updated.

Note:

const doesn't mean immutable as properties in a const object can be changed. To avoid confusion I suggest using let for objects.

[agentlame]

I understand const. But how does let differ from var?

[creesch]

From mdn

let allows you to declare variables that are limited in scope to the block, statement, or expression on which it is used. This is unlike the var keyword, which defines a variable globally, or locally to an entire function regardless of block scope.

In other words, let is a bit more specific and should prevent some reassignment bugs and that.