Inconsistency with the existential operator

[michaelowens]

Quicky shown in one code:

if varname? then
varname2 = "hi"
if varname2? then
delete varname2
if varname2? then

Compiles to

var varname2;

if (typeof varname !== "undefined" && varname !== null) {}

varname2 = "hi";

if (varname2 != null) {}

delete varname2;

if (varname2 != null) {}

The last one doesn't check for undefined, even though the variable was deleted. I think it's bad for CoffeeScript to decide to not check for undefined based on how the script was during compiling.

[yuchi]

Luckily enough it's not a problem in JS where you _cannot delete vars_!

Have a look at @kangax's http://perfectionkills.com/understanding-delete/ awesome article.

tl;dr: 'cause in JS vars are not deleted checking for varA != null is valid even after a delete varA; (which is not successful).

[michaelficarra]

Thanks to #2021, that syntax won't even be allowed anyway. But yeah, as @yuchi says, delete won't remove anything from scope that was added in a variable declaration, as all variables (known to the scope check) in coffeescript are. Not a bug. Feel free to re-open if you can find a way for an existentially-guarded reference to produce a ReferenceError.