number().min(1).max(5) is equivalent to the open interval [1,5] (1, 2, 3, 4 and 5 are valid numbers, as well as floating point numbers in that range).

However, there doesn't seem to be a way to declare closed intervals?!

Suggested addition:
number().minExclusive(1).maxExclusive(5) would be the equivalent of (1,5). Valid numbers would be 2.0, 3.0, 4.0, but also 1.1 and 4.999. 1.0 and 5.0 are excluded.

This is only useful if floating point numbers are allowed of course, and not for number().integer().