Default argument values in JavaScript functions

Possible Duplicate:
How do I make a default value for a parameter to a javascript function

in PHP:

function func($a = 10, $b = 20){
  // if func() is called with no arguments $a will be 10 and $ b  will be 20
}

How can you do this in JavaScript?

I get a error if I try to assign values in function arguments

missing ) after formal parameters