How do I set an optional argument in a Pug mixin?

Question

Pug

There is a mixin:

mixin select(length=5)
  select
    - for (var i = 1; i <= length; i++)
      option(value=i)= i

It needs to accept a class as an optional argument.
For example when calling:
+select(class="my-class")

Answer 1

mixin select(value, length=5, elClass)
  - elClass = elClass || ''
  select(class = elClass)
    - for (var i = 1; i <= length; i++)
      option(value=i)= i

Answer 2

Everything is in the help starting from here .
Internally, it can be used with or without explicit binding.