Why is the function argument "x" not set to "undefined" when using "x?"

Question

The following CoffeeScript code :

foo = (x) -> alert("hello") unless x? alert("world") unless y? 

compiled for:

 var foo; foo = function(x) { if (x == null) { alert("hello"); } if (typeof y === "undefined" || y === null) { return alert("world"); } }; 

Why is foo x not set to undefined and y is not?

Answer 1

An undefined check is to throw a ReferenceError exception when you retrieve the value of a nonexistent identifier:

 >a == 1 ReferenceError: a is not defined 

The compiler can see that the identifier x exists, because this is an argument to the function.

The compiler cannot determine if the identifier y exists, and therefore it is necessary to check if y exists.

 // y has never been declared or assigned to >typeof(y) == "undefined" true