What techniques can be used to define a class in J

Question

I prefer to use OOP in large scale projects like the one I'm working on right now. I need to create several classes in JavaScript but, if I'm not mistaken, there are at least a couple of ways to go about doing that. What would be the syntax and why would it be done in that way?

I would like to avoid using third-party libraries - at least at first.
Looking for other answers, I found the article Object-Oriented Programming with JavaScript, Part I: Inheritance - Doc JavaScript that discusses object-oriented programming in JavaScript. Is there a better way to do inheritance?

Answer 1

I prefer to use Daniel X. Moore's {SUPER: SYSTEM}. This is a discipline that provides benefits such as true instance variables, trait based inheritance, class hierarchies and configuration options. The example below illustrates the use of true instance variables, which I believe is the biggest advantage. If you don't need instance variables and are happy with only public or private variables then there are probably simpler systems.

function Person(I) {
  I = I || {};

  Object.reverseMerge(I, {
    name: "McLovin",
    age: 25,
    homeState: "Hawaii"
  });

  return {
    introduce: function() {
      return "Hi I'm " + I.name + " and I'm " + I.age;
    }
  };
}

var fogel = Person({
  age: "old enough"
});
fogel.introduce(); // "Hi I'm McLovin and I'm old enough"

Wow, that's not really very useful on it's own, but take a look at adding a subclass:

function Ninja(I) {
  I = I || {};

  Object.reverseMerge(I, {
    belt: "black"
  });

  // Ninja is a subclass of person
  return Object.extend(Person(I), {
    greetChallenger: function() {
      return "In all my " + I.age + " years as a ninja, I've never met a challenger as worthy as you...";
    }
  });
}

var resig = Ninja({name: "John Resig"});

resig.introduce(); // "Hi I'm John Resig and I'm 25"

Another advantage is the ability to have modules and trait based inheritance.

// The Bindable module
function Bindable() {

  var eventCallbacks = {};

  return {
    bind: function(event, callback) {
      eventCallbacks[event] = eventCallbacks[event] || [];

      eventCallbacks[event].push(callback);
    },

    trigger: function(event) {
      var callbacks = eventCallbacks[event];

      if(callbacks && callbacks.length) {
        var self = this;
        callbacks.forEach(function(callback) {
          callback(self);
        });
      }
    },
  };
}

An example of having the person class include the bindable module.

function Person(I) {
  I = I || {};

  Object.reverseMerge(I, {
    name: "McLovin",
    age: 25,
    homeState: "Hawaii"
  });

  var self = {
    introduce: function() {
      return "Hi I'm " + I.name + " and I'm " + I.age;
    }
  };

  // Including the Bindable module
  Object.extend(self, Bindable());

  return self;
}

var person = Person();
person.bind("eat", function() {
  alert(person.introduce() + " and I'm eating!");
});

person.trigger("eat"); // Blasts the alert!

Disclosure: I am Daniel X. Moore and this is my {SUPER: SYSTEM}. It is the best way to define a class in JavaScript.

Answer 2

If you're going for simple, you can avoid the "new" keyword entirely and just use factory methods. I prefer this, sometimes, because I like using JSON to create objects.

function getSomeObj(var1, var2){
  var obj = {
     instancevar1: var1,
     instancevar2: var2,
     someMethod: function(param)
     {  
          //stuff; 
     }
  };
  return obj;
}

var myobj = getSomeObj("var1", "var2");
myobj.someMethod("bla");

I'm not sure what the performance hit is for large objects, though.

Answer 3

var Student = (function () {
    function Student(firstname, lastname) {
        this.firstname = firstname;
        this.lastname = lastname;
        this.fullname = firstname + " " + lastname;
    }

    Student.prototype.sayMyName = function () {
        return this.fullname;
    };

    return Student;
}());

var user = new Student("Jane", "User");
var user_fullname = user.sayMyName();

Thats the way TypeScript compiles class with constructor to JavaScript.

Answer 4

You probably want to create a type by using the Folding Pattern:

    // Here is the constructor section.
    var myType = function () {
        var N = {}, // Enclosed (private) members are here.
            X = this; // Exposed (public) members are here.

        (function ENCLOSED_FIELDS() {
            N.toggle = false;
            N.text = '';
        }());

        (function EXPOSED_FIELDS() {
            X.count = 0;
            X.numbers = [1, 2, 3];
        }());

        // The properties below have access to the enclosed fields.
        // Careful with functions exposed within the closure of the
        // constructor, each new instance will have it's own copy.
        (function EXPOSED_PROPERTIES_WITHIN_CONSTRUCTOR() {
            Object.defineProperty(X, 'toggle', {
                get: function () {
                    var before = N.toggle;
                    N.toggle = !N.toggle;
                    return before;
                }
            });

            Object.defineProperty(X, 'text', {
                get: function () {
                    return N.text;
                },
                set: function (value) {
                    N.text = value;
                }
            });
        }());
    };

    // Here is the prototype section.
    (function PROTOTYPE() {
        var P = myType.prototype;

        (function EXPOSED_PROPERTIES_WITHIN_PROTOTYPE() {
            Object.defineProperty(P, 'numberLength', {
                get: function () {
                    return this.numbers.length;
                }
            });
        }());

        (function EXPOSED_METHODS() {
            P.incrementNumbersByCount = function () {
                var i;
                for (i = 0; i < this.numbers.length; i++) {
                    this.numbers[i] += this.count;
                }
            };
            P.tweak = function () {
                if (this.toggle) {
                    this.count++;
                }
                this.text = 'tweaked';
            };
        }());
    }());

That code will give you a type called myType. It will have internal private fields called toggle and text. It will also have these exposed members: the fields count and numbers; the properties toggle, text and numberLength; the methods incrementNumbersByCount and tweak.

The Folding Pattern is fully detailed here: Javascript Folding Pattern

Answer 5

JavaScript is object-oriented, but it's radically different than other OOP languages like Java, C# or C++. Don't try to understand it like that. Throw that old knowledge out and start anew. JavaScript needs a different thinking.

I'd suggest to get a good manual or something on the subject. I myself found ExtJS Tutorials the best for me, although I haven't used the framework before or after reading it. But it does give a good explanation about what is what in JavaScript world. Sorry, it seems that that content has been removed. Here's a link to archive.org copy instead. Works today. :P

Answer 6

Object Based Classes with Inheritence

var baseObject = 
{
     // Replication / Constructor function
     new : function(){
         return Object.create(this);   
     },

    aProperty : null,
    aMethod : function(param){
      alert("Heres your " + param + "!");
    },
}


newObject = baseObject.new();
newObject.aProperty = "Hello";

anotherObject = Object.create(baseObject); 
anotherObject.aProperty = "There";

console.log(newObject.aProperty) // "Hello"
console.log(anotherObject.aProperty) // "There"
console.log(baseObject.aProperty) // null

Simple, sweet, and gets 'er done.