Constructors in Javascript objects

Question

Can Javascript classes/objects have constructors and how are they created? Any examples?

Answer 1

Here are a couple of excellent explanations:

• http://www.javascriptkit.com/javatutors/oopjs2.shtml
• http://www.c-point.com/javascript_tutorial/constructor.htm

Answer 2

http://msdn.microsoft.com/en-us/library/202863ha%28VS.85%29.aspx

Answer 3

If you have an hour or so this is an excellent lecture the covers this topic.
Douglas Crockford: "Advanced JavaScript"

Answer 4

Here's a template I sometimes use for OOP-similar behavior in JavaScript. As you can see, you can simulate private (both static and instance) members using closures. What new MyClass() will return is an object with only the properties assigned to the this object and in the prototype object of the "class."

var MyClass = (function () {
    // private static
    var nextId = 1;

    // constructor
    var cls = function () {
     // private
     var id = nextId++;
     var name = 'Unknown';

     // public (this instance only)
     this.get_id = function () { return id; };

     this.get_name = function () { return name; };
     this.set_name = function (value) {
      if (typeof value != 'string')
       throw 'Name must be a string';
      if (value.length < 2 || value.length > 20)
       throw 'Name must be 2-20 characters long.';
      name = value;
     };
    };

    // public static
    cls.get_nextId = function () {
     return nextId;
    };

    // public (shared across instances)
    cls.prototype = {
     announce: function () {
      alert('Hi there! My id is ' + this.get_id() + ' and my name is "' + this.get_name() + '"!\r\n' +
            'The next fellow\'s id will be ' + MyClass.get_nextId() + '!');
     }
    };

    return cls;
})();

Answer 5

Click Upvote, your code sample is wrong. You're alerting an object, not the colour. Also the getColor function is defined locally to the constructor, not to the class.

function Box(color)
{
   this.color=color;

   this.getColor = function()
   {
       return this.color;
   }
}

var blueBox=new Box("blue");
alert(blueBox.getColor()); //wlll print blue

var greenBox=new Box("green");
alert(greenBox.getColor()); //wlll print green