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This method is provided to ease the task of debugging JavaScript.
It provides a general purpose way to call an unconnected function when a property value is changed. The function does not need to be called explicitly. The function that gets called has a particular API, which passes the following values:
Property name
Old value
New value
It gets an opportunity to modify the new value or veto the change by returning the old value. Whatever value is returned is stored in the property. You can carry out other JavaScript tasks during this property call, although it is probably to best avoid making changes to other property values with watch-points that call the same handler, because you could set up a recursive loop.
If you invoke the watch() method without specifying a receiving object, as if it were a function, you are actually setting watch-points on global object properties. Since this is where global variables live, you can monitor them as easily as object properties.
The new event model supported by Netscape 6.0 and that already available in MSIE 5.0 present a propertyName property that belongs to the Event object. You can inspect that during an onPropertyChanged event and achieve the same watch()/unwatch() behavior.
<HTML>
<HEAD>
</HEAD>
<BODY>
<SCRIPT>
// Code works with Netscape 4+ only
// Define initial value for property
var XXX = 10;
// Define watch handler function
function watchHandler(aProp, anOldVal, aNewVal)
{
var myText = "";
myText += "Property name ...: ";
myText += aProp;
myText += "\n";
myText += "Old value .......: ";
myText += anOldVal;
myText += "\n";
myText += "New value .......: ";
myText += aNewVal;
myText += "\n";
alert(myText);
return aNewVal;
}
// Register the watch handler
watch("XXX", watchHandler);
</SCRIPT>
Some body text
<SCRIPT>
// Modify the property to trigger the watch handler
XXX = 1000;
</SCRIPT>
</BODY>
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JavaScript Programmer's Reference, Cliff Wootton Wrox Press (www.wrox.com) Join the Wrox JavaScript forum at p2p.wrox.com Please report problems to support@wrox.com © 2001 Wrox Press. All Rights Reserved. Terms and conditions. | ||