How do I use the CoffeeScript existential operator to check some object properties for undefined?

Question

I would like to use the CoffeeScript existential operator to check some object properties for undefined. However, I encountered a little problem.

Code like this:

Answer 1

This is a common point of confusion with the existential operator: Sometimes

x?

compiles to

typeof test !== "undefined" && test !== null

and other times it just compiles to

x != null

The two are equivalent, because x != null will be false when x is either null or undefined. So x != null is a more compact way of expressing (x !== undefined && x !== null). The reason the typeof compilation occurs is that the compiler thinks x may not have been defined at all, in which case doing an equality test would trigger ReferenceError: x is not defined.

In your particular case, test.test may have the value undefined, but you can't get a ReferenceError by referring to an undefined property on an existing object, so the compiler opts for the shorter output.