JavaScript 2.0-The Complete Reference, Second ... - freecodingtutorial

to detect if Internet Explorer is in use. However, does the existence of document.all really
mean that Internet Explorer is in use? The truth of the matter is that another browser could
support document.all but not necessarily provide all the features found in Internet Explorer.
The developer might even be simulating document.all with their own code. Given all the
possibilities for trouble, it might be better to check for each object specifically, so instead we
might use
var allObject = (document.all) ? true : false;
var getById = (document.getElementById) ? true : false;
and so on. In some ways, object detection is the best method to use, but it should be used
carefully and assumptions shouldn‘t be made.
Another consideration with object detection is not to go too far too quickly. Remember that
probing a property of a nonexistent object throws an error, so first check to see if the object
exists. As an example, if you were checking for window.screen.height and you just did
if (window.screen.height)
// do something
you would throw an error in browsers that did not support the Screen object. Instead you could
rely on short-circuit evaluation to do the test incrementally, like so:
if (window.screen && window.screen.height)
// do something
Advanced JavaScript programmers might see that the object detection approach fits nicely with
try/catch blocks.
Java Detection
Detecting Java‘s availability is fairly easy using the Navigator method javaEnabled(). This
method returns true if Java is available and turned on, and false otherwise.
if (navigator.javaEnabled())
else
// do Java stuff or write out tag
alert("Sorry no Java");
You can find out more about Java once you know it is available by accessing a Java applet
included in the page. You can even determine what type of Java Virtual Machine is supported.
In order to do this, you will have to access the public methods and properties of a Java applet.
Interacting with applets is discussed in more detail in Chapter 18.
Plug-in Detection
In Netscape 3+ (and Opera 4+), each plug-in installed in the browser has an entry in the
plugins[] array of the Navigator object. Each entry in this array is a Plugin object containing
information about the specific vendor and version of the component. A simple detection scheme
checks for a plug-in‘s existence using the associative array aspect of JavaScript collections. For
example, to look for a Flash plug-in, you might write
if (navigator.plugins["Shockwave Flash"])