How does CoffeeScript's existential operator work? - coffeescript

Question

Coffeescript uses the existential operator to determine when a variable exists, and in the coffeescript documentation it shows that something? would compile to something !== undefined && something !== null however I noticed that my version of coffeescript was only compiling this to something !== null so I wrote a test to see how this would effect my code
taco = undefined
if taco?
console.log "fiesta!"
else
console.log "No taco!"
which compiled to
// Generated by CoffeeScript 1.4.0
(function() {
var taco;
taco = void 0;
if (taco != null) {
console.log("fiesta!");
} else {
console.log("No taco!");
}
}).call(this);
and outputted the somewhat unexpected No taco! so my question is two fold. Why does coffeescript no longer check for the value being undefined and why is this suficiant?

Answer 1

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.