How to have a statement identify an undefined variable? [duplicate]

Let's say you have the following function:

var variable; function(variable); function function(variable) { alert ("variable equals " + variable); if (variable != 'undefined') { ~~~code1~~~ } else { ~~~code2~~~ } } 

The alert outputs:

variable equals undefined

However, ~~~code2~~~ is never executed. I'm guessing that my syntax is incorrect. If I haven't defined the variable, how do I get the function function to execute ~~~code2~~~?

Extra Information

When the variable variable is hardcoded, as in the following code:

var variable; function(variable) function function(variable) { variable = 2; alert ("variable equals " + variable); if (exponent == 2) { ~~~code1~~~ } else { ~~~code2~~~ } } 

~~~code1~~~ is executed.