Working with custom JavaScript objects

Most of the time you will be referencing objects which are built-in to the DOM. However, you may want to create your own objects for storing data within a JavaScript program. There are several ways to create a new object, but we'll look at two:

creating a direct instance of an object
creating an object prototype.

Direct instance of an object

Direct instance of an object simply means creating a new single object, such as point:

var point = new Object();
point.x = 45;
point.y = -3;
point.title = "Location";

Assigning a method to your new object is also simple. Assume that you already have coded a function named format, which returns a string representation of that object in the form title: x y. To assign this function as a method toString() of your object, use the syntax:

point.toString = format;

Prototype of an object

Sometimes, you'll want to create a "template" or prototype of an object. This does not create an actual instance of the object, but defines the structure of the object. In the future, then, you can quickly stamp out a particular instance of the object. Suppose that instead of point, you created a prototype object named Point. This object could then be a template for a particular point object. First, create a function which defines the object structure:

function Point(x, y, title) 
{ 
  this.x = x;
  this.y = y;
  this.title = title;

  this.toString = function()
  {
    return(title + ": " + x + " " + y);
  }
}

As soon as the Point prototype has been set, you can quickly create single instances of that object by using the keyword new:

var p1 = new Point(3, -6, "Location");
var p2 = new Point(0, 0, "Origin");

By creating the object instances, it is perfectly legal not to pass one or more tailing parameters to its constructor. This is a general rule by calling any JavaScript function. Thus, the following code

var p = new Point(5, 3);
document.write(p.toString());
// document.write(p); // alternative code

produces the output undefined: 5 3.

printTitle()