Question

I’m very new in coffeescript and also with objects in javascript.
I have this piece of code in coffeescript:

class Animal
constructor: (name) ->
    @name = name
    @upperName: ->
        @name.toUpperCase()
    @greetings ->
        console.log 'Hello %s', @name

this.type = 'Animal'

That is “compiled” into this javascript:

var Animal
Animal = (function() {
function Animal(name) {
  this.name = name;
  ({
    this.upperName: function() {
      return this.name.toUpperCase();
    }
  });
  this.greetings(function() {
    return console.log('Hello %s', this.name);
  });
}
Animal.type = 'Animal';
return Animal;
})();

What’s the difference between the methods greetings and upperName???
What the “:” do in a class?

Thanks

Answer 1

Symbol summary (left=CS, right=JS)

Within class Animal:

identifier: value        Animal.prototype.identifier = value
@identifier: value       Animal.identifier           = value
@identifier= value       Animal.identifier           = value
identifier = value       identifier                  = value  (private var)

Elsewhere (ordered by same compiled result)

Animal::identifier = value  Animal.prototype.identifier = value
Animal.identifier  = value  Animal.identifier           = value
identifier = value          identifier                  = value
// New:
@identifier = value         this.identifier             = value
identifier: value           { identifier: value}                 (object literal)
@identifier: value          ---INVALID---

---

In CoffeeScript, @ compiles to this.

In the context of a class construct, the method definition is affected by use of @ (this). Here’s a simple example:

class ClassObject
    instanceMethod: ->
        # This method will be defined on the prototype (available to instance)

    @classMethod: ->
        # This method is defined on the class object itself, not on the instance

    this.classMethod2 = -> # See previous and below
    privateVar = "private"

Although the syntax slightly differs, the latest two have the same compiled result.

"What does : mean inside a class block?"

It is used to define properties. When = (is equal sign) is used instead, a "private" variable will be defined.

"What does : mean inside the (constructor) method?

Outside the level of a class (eg top-level code, inside a function, constructor, etc.), : does not have the "special class" meaning. : is the separator between key-name pairs within an object literal.
Your given code, @upperName: -> ... is invalid, and does not compile in the latest CoffeeScript version. upperName: -> ... is valid though, and will compile to an object literal with property upperName and a function as a value.

---

Have a look at the compiled CoffeeScript code:

var ClassObject;

ClassObject = (function() {
  var privateVar;

  function ClassObject() {}

  ClassObject.prototype.instanceMethod = function() {};

  ClassObject.classMethod = function() {};

  ClassObject.classMethod2 = function() {};

  privateVar = "private";

  return ClassObject;

})();