Using JavaScript, create a class that converts an infix expression to a postfix expression. An infix expression, for example, is "3 + 4", while a postfix expression is "3 4 +". The class should accept a string as input and be able to handle the following: any of the four main operators (+, -, *, /), any number (including numbers with more than one digit), and parentheses.
Examples of potential inputs:
1+1
(11-4)
(33*4-(2+5))*4/2
Then, create another class that evaluates the postfix expression and returns an integer result.