Statement inside "IF" like a variable...?

Greetings,
I don’t know if this question is somewhat crazy but I’d like to make the statement inside an IF conditional, working like a variable.

Example:

 
var a = 2;
var b = 4;
 
var Expression = "a<0 && b >5"
 
if Expression true then...

Just like a query system for a database.
How could achieve this? Do I need to build a “special” interpreter for this?