Coffeescript ||= analogue?

Question

I'm primarily a Rails developer, and so in whipping up a little script for my company's Hubot instance, I was hoping to accomplish the following:

robot.brain.data.contacts ||= {} 

Or, only make this new hash if it doesn't already exist. The idea being that I want to have a contacts array added dynamically through the script so I don't have to modify Hubot's source, and I obviously don't want to overwrite any contacts I add to it.

Question: is there a quick little construct like the Rails ||= that I can use in Coffeescript to achieve the above goal?

Cheers.

Answer 1

You can use ?= for conditional assignment:

speed ?= 75 

The ? is the "Existential Operator" in CoffeeScript, so it will test for existence (not truthiness):

if (typeof speed === "undefined" || speed === null) speed = 75; 

The resulting JS is a bit different in your case, though, because you are testing an object property, not just a variable, so robot.brain.data.contacts ?= {} results in the following:

var _base, _ref; if ((_ref = (_base = robot.brain.data).contacts) != null) {   _ref; } else {   _base.contacts = {}; }; 

More info: http://jashkenas.github.com/coffee-script/

Answer 2

I personally use or= instead of ?= mainly because that's what I call ||= (or-equal) when I use it in Ruby.

robot.brain.data.contacts or= {} 

The difference being that or= short-circuits when robot.brain.data.contacts is not null, whereas ?= tests for null and only sets robot.brain.data.contacts to {} if not null.

See the compiled difference.

As mentioned in another post, neither method checks for the existence of robot, robot.brain or robot.brain.data, but neither does the Ruby equivalent.

Edit:

Also, in CoffeeScript or= and ||= compile to the same JS.