How to Use JavaScript++ WebReference.com- How to Create A JavaScript Web Page Screen Saver | WebReference

How to Use JavaScript++ WebReference.com- How to Create A JavaScript Web Page Screen Saver

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How to Use JavaScript++

This article was inspired by some work I did for a web site a few years ago. One problem I encountered was that many of the core features of JavaScript that I wanted to use were only supported in the latest browsers.

 

At the time, IE6.0 was just being released and my requirements were to design the website to support browsers as old as IE4.0 and Netscape 4.7. In a landscape as rocky and variable as JavaScript with the existing browsers the maxim is often: "write once, pray it runs on most browsers."

 

The usual mantra is "program to the lowest common denominator" but the problem was that these browsers had little in common. It would be nice to believe that most people have upgrated to the latest browsers, but in reality, there are many old browsers still surfing the net. A quick visit to www.thecounter.com indicates that roughly 80% of users are using the latest browsers, 20% are not. That's a lot of users. To solve the problem, I had to plug the holes, factor up the denominators or – add something to JavaScript.

Some Background First

To the JavaScript developer, JavaScript objects can be thought of as an amorphous blob of un-typed properties. Every detail of the object is stored in this manner, including member variables, constructors, methods and even array items.

 

JavaScript is most certainly object based; you can't go far in JavaScript without bumping into an object of some kind. There are a fair number of different types predefined in the language; Array objects store items indexed by numbers and come with some useful methods, Date objects provide useful methods for measuring time and so on. Even numbers are sometimes converted into objects when necessary.

 

JavaScript ‘Classes' are based on a special property called prototype. The prototype contains predefined properties and functions that are common to all instances of the particular class. Prototypes are held in a chain for each object and this offers a type of inheritance. A Date object automatically has all the properties and methods defined in Date.prototype as well as those defined in Object.prototype.

Now for the details…

Now that we know a bit about JavaScript objects, prototypes and properties, let's have a look at how we can solve the original problem stated above.

The undefined property

A relatively recent addition to JavaScript is the undefined property. It's useful when you want to test whether a variable has been initialized or not.

 

var a;

if ( a == undefined ) a = "some value";

 

The trouble with this code is that if JavaScript doesn't support the undefined property, the code above will fail with an error.

The solution is to simply declare the undefined property in the code…

 

var undefined;

 

This variable will be added to the Global object and since it is not given any value, its value will be undefined. You don't need to worry about the newer browsers that already support the undefined property, this code replaces the predefined version and behaves in the same way.

New Array methods

Arrays have been supported in JavaScript for some time, however a few new methods have been recently added, including push, pop, shift, splice and unshift.

We'll start with push and pop…

 

// toNumber() – a convenience function to convert a variable to a number.

Array.prototype.toNumber = function(x)

{

   // return the numerical value of x only if it is completely numeric

   switch ( typeof(x) )

   {

      case "number":

         return x;

      case "string":

         var n = parseInt(x);

         if ( !isNaN(n) && ("A"+n == "A"+x) ) return n;

         break;

      default:

         break;

   }

   // otherwise return undefined

}

// highestIndex() – convenience function to return the highest numerical index

// in an array, or undefined if none are found.

Array.prototype.highestIndex = function()

{

   var m;

   // find the highest array index and add a next one

   for ( var i in this )

   {

      var n = this.toNumber(i);

      if ( (n != undefined) && ((m == undefined) || (n > m) ) ) m = n;

   }

   return m;

}

// only add a push method if Array doesn't already have one

if ( !Array.prototype.push ) Array.prototype.push = function()

{

   var m = this.highestIndex();

   if ( m == undefined ) m = -1;

   for ( var i = 0; i < arguments.length; i++ )

      this[++m]=arguments[i];

}

// only add a pop method if Array doesn't already have one

if ( !Array.prototype.pop ) Array.prototype.pop = function()

{

   var m = this.highestIndex();

   var v;

   if ( m != undefined )

   {

      v = this[m];

      delete this[m];

   }

   return v;

}

 

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Created: March 27, 2003
Revised: September 29, 2004

URL: http://webreference.com/programming/javascript/gr/column9/1