How does CoffeeScript's existential operator work?

The documentation says this about ?:

CoffeeScript's existential operator ? returns true unless a variable is null or undefined, which makes it analogous to Ruby's nil?

so of course this will say "No taco!":

taco = undefined 
if taco?
    console.log "fiesta!"
else 
    console.log "No taco!"

Your taco is explicitly undefined so taco? is false.

CoffeeScript implicitly declares variables so the JavaScript form of ? is context dependent. For example, if you just say only this:

if taco?
    console.log "fiesta!"
else 
    console.log "No taco!"

you'll see that taco? becomes typeof taco !== "undefined" && taco !== null. You still see the "is it null" check (in a tighter form) but there's also the "is there a var taco" check with typeof; note that the typeof taco test also checks for taco = undefined so a stricter !== test can be used to see if taco is null.

You say this:

I noticed that my version of coffeescript was only compiling this to something !== null

but that's not what it is doing, it is actually compiling to something != null; note the use of "sloppy" type converting inequality (!=) versus the strict inequality (!==) that you claim is there. The difference between != and !== is important here since:

• Null and Undefined Types are == (but not ===)

So if you know that variable v has been declared (i.e. there is var v somewhere) then v != null is sufficient to check that v is neither null nor undefined. However, if you do not know that v has been declared, then you need a typeof check to avoid a ReferenceError when you try to compare an undeclared variable with null. Consider this JavaScript:

if(taco != null) {
    console.log("fiesta!");
} else {
    console.log("No taco!");
}

That will throw a ReferenceError in your face since taco does not exist. This:

if(typeof taco !== "undefined" && taco !== null)
    console.log("fiesta!");
} else {
    console.log("No taco!");
}

on the other hand is fine since the typeof check guards against trying to access something that hasn't been declared. I don't think you can construct the first one in CoffeeScript without embedding JavaScript using backticks.