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Unintuitive E4X Gotcha in Actionscript 3

In Weblog on 8 January 2008 tagged as3, e4x, Programming, xml with no comments

This is the second in a series of posts about actionscript 3 that I announced earlier, and my last for today. Here's the feed of posts tagged as3.

Testing for the existence of a property on an Object is simple in actionscript 3:

 
 
var object:Object = { sub: "sub" };
 
trace("testing object properties");
 
if (object.sub) {
	trace("CORRECT: object has a sub element");
}
else {
	trace("WRONG: object has no sub element");
}
 
if (object.nosub) {
	trace("WRONG: object has a nosub element");
}
else {
	trace("CORRECT: object has no nosub element");
}
 

This yields the correct output:

testing object properties
CORRECT: object has a sub element
CORRECT: object has no nosub element

However, the same thing isn't as intuitive with XML and E4X:

 
 
var xml:XML = <xml><sub>sub</sub></xml>;
 
trace("testing xml element");
 
// don't do this, it will give false positives!
if (xml.sub) {
	trace("CORRECT: xml has a sub element");
}
else {
	trace("WRONG: xml has no sub element");
}
 
// don't do this, it will give false positives!
if (xml.nosub) {
	trace("WRONG: xml has a nosub element");
}
else {
	trace("CORRECT: xml has no nosub element");
}
 

This gives incorrect output:

testing xml element
CORRECT: xml has a sub element
WRONG: xml has a nosub element

The way to test reliably for the existence of xml elements is to check the length() method on the element:

 
 
trace("testing xml length()");
 
// this works to test for the existence of an element
if (xml.sub.length()) {
	trace("CORRECT: xml has a sub element");
}
else {
	trace("WRONG: xml has no sub element");
}
 
// this also works to test for the non-existence of an element
if (xml.nosub.length()) {
	trace("WRONG: xml has a nosub element");
}
else {
	trace("CORRECT: xml has no nosub element");
}
 

This yields the correct output:

testing xml length()
CORRECT: xml has a sub element
CORRECT: xml has no nosub element

Hey, it's boring but true!