Hard Rock CoffeeScript

Operators and aliases

CoffeeScript includes some useful aliases to provide you with shorthands. First one is @, which is an alias for this keyword in JavaScript.

CoffeeScript

goByGuitar = ->
  @guitar = true

JavaScript

var goByGuitar;
goByGuitar = function() {
  return this.guitar = true;
};

Another useful alias shortens your expression is ::, which is an alias for prototype.

CoffeeScript

Band::members = -> @members.length

JavaScript

Band.prototype.members = function() {
  return this.members.length;
};

Because the == operator frequently causes undesirable coercion, is intransitive, and has a different meaning than in other languages, CoffeeScript compiles == into ===, and != into !==.

CoffeeScript also provides some human readable aliases for this operators: you can use is instead of == and isnt instead of !=. And also you can use not as an alias for !.

CoffeeScript

'life' is 'life' # true
'iron man' isnt 'sandman' # true
true is not false # true

JavaScript

'life' === 'life';
'iron man' !== 'sandman';
true === !false;

Logical operators aliases are pretty straightforward - and compiles to &&, and or into ||. You can also use in to test for array presence, and of to test for JavaScript object-key presence.

The Existential Operator

It's a little difficult to check for the existence of a variable in JavaScript. if (variable) ... comes close, but fails for zero, the empty string, and false.

Existence

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

Behind the scenes CoffeeScript will check previously defined variables like this:

variable != null;

and if variable haven't been defined previously like this:

typeof variable !== "undefined" && variable !== null;

Conditional assignment

It can also be used for safer conditional assignment than ||= provides, for cases where you may be handling numbers or strings.

band = null
band ?= 'Stone Sour'

Or operator

You can also use it in place of the || operator:

band = missingName ? 'Halloween'

Null check

You can skip a null check before accessing a property by placing the existential operator right before it. Use ?. instead of the dot accessor . in cases where the base value may be null or undefined. If all of the properties exist then you'll get the expected result, if the chain is broken, undefined is returned instead of the TypeError that would be raised otherwise.

CoffeeScript

singer.playTheGuitar()?.solo()

JavaScript

var _ref;
if ((_ref = singer.playTheGuitar()) != null) {
  _ref.solo();
}

Callable check

Similarly you can check that a property is actually a function, and callable, by placing the existential operator right before the parens. If the property doesn't exist, or isn't a function, it simply won't get called.

CoffeeScript

guitarist.sing().chorus?()

JavaScript

var _base;
if (typeof (_base = guitarist.sing()).chorus === "function") {
  _base.chorus();
}