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Liam McLennan hackingon.net

Recently I wrote about JavaScript class patterns, and in particular, my favourite class pattern that uses closure to provide encapsulation. A class to represent a person, with a name and an age, looks like this:

var Person = (function() {
  // private variables go here
  var name,age;
  
  function constructor(n, a) {
    name = n;
    age = a;
  }

  constructor.prototype = {
    toString: function() {
      return name + " is " + age + " years old.";
    }
  };
  
  return constructor;  
})();

var john = new Person("John Galt", 50);
console.log(john.toString());

Today I have been experimenting with coding for node.js in CoffeeScript. One of the first things I wanted to do was to try and implement my class pattern in CoffeeScript and then see how it compared to CoffeeScript’s built-in class keyword. The above Person class, implemented in CoffeeScript, looks like this:

# JavaScript style class using closure to provide private methods
Person = (() ->
	[name,age] = [{},{}]
		
	constructor = (n, a) ->
		[name,age] = [n,a]
		null
		
	constructor.prototype = 
		toString: () ->
			"name is #{name} age is #{age} years old"
	
	constructor
)()

I am satisfied with how this came out, but there are a few nasty bits. To declare the two private variables in javascript is as simple as var name,age; but in CoffeeScript I have to assign a value, hence [name,age] = [{},{}]. The other major issue occurred because of CoffeeScript’s implicit function returns. The last statement in any function is returned, so I had to add null to the end of the constructor to get it to work.

The great thing about the technique just presented is that it provides encapsulation ie the name and age variables are not visible outside of the Person class. CoffeeScript classes do not provide encapsulation, but they do provide nicer syntax. The Person class using native CoffeeScript classes is:

# CoffeeScript style class using the class keyword
class CoffeePerson
	constructor: (@name, @age) ->
	
	toString: () ->
		"name is #{@name} age is #{@age} years old"
		
felix = new CoffeePerson "Felix Hoenikker", 63
console.log felix.toString()

So now I have a trade-off: nice syntax against encapsulation. I think I will experiment with both strategies in my project and see which works out better.

Posted on Friday, March 18, 2011 7:47 PM | Back to top

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