Replace Var with Let and Const
var with block-scoped variables
const are block-scope variable declarations that can replace 'var' declarations in many cases.
const declares blocked-scoped variables that cannot be re-assigned.
let declares block-scoped variables that can be changed.
They are available since ES6 and are preferred over function-scope, modifiable
var declarations, because they make it easier to reason about the code.
When there are several variables declared in a statement, this refactoring does not change the declaration to prevent formatting breakages and linter issues.