CoffeeScript's existential operator ? check if the variable is null or undefined.

1. Check for null or undefined.

alert "Hello CoffeeScript!" if myVar?

javascript equivalent:

if (typeof myVar !== "undefined" && myVar !== null) {
  alert("Hello CoffeeScript!");
}

2. Safer conditional assignment

You can also use this operator safer conditional assignment

language = favoriteLanguage ? "coffeescript"

javascript equivalent:

language = typeof favoriteLanguage !== "undefined" && favoriteLanguage !== null ? favoriteLanguage : "coffeescript";

3. Safe chaining of methods

Instead of chaining the methods with . chain them with ?. to avoid raising the TypeError.

firstName = user?.profile?.firstname

javascript equivalent:

firstName = typeof user !== "undefined" && user !== null ? (ref = user.profile) != null ? ref.firstname() : void 0 : void 0;

If all of the properties exist then you'll get the expected result if the chain is broken, undefined is returned

CoffeeScript	JavaScript
is, ==	===
isnt, !=	!==
not	!
and	&&
or	||
true, yes, on	true
false, no, off	false
@, this	this
of	in
in	No equivalent
a ** b	Math.pow(a, b)
a // b	Math.floor(a / b)
a %% b	(a % b + b) % b