ActionScriptEvery new version of ActionScript has had its impact on the compiler and error handling. Since ActionScript 1, that had loose typing and was suited well for small-scale scripting projects, things have changed considerably.

With ActionScript 2 Flash community first saw the compile-time type checking and class-based syntax which basically allowed Object-oriented programming approaches. Despite the class-based inheritance that took AS2 closer to Java for instance, compiling still took it to AS1 bytecode over legacy matters. So basically the class-based inheritance that allowed the OOP approach was a layer on top of the existing prototype-centric AS1.

With Flex 2 and ActionScript 3 the scripting language changed a lot. The whole thing was restructured and new AS3-specific virtual machine (AVM2) was introduced. For those working in Flash IDE prior this era, it changed a lot. With a "habit" of procedural programming they had to rethink it. On the new platform, even if compile-time errors made sense referring to the line error occurred at, run-time errors didn't really help them out much, e.g.:

TypeError: Error #1009: Cannot access a property or method of a null object reference.

or slightly better

ReferenceError: Error #1069: Property property0 not found on Doc and there is no default value.

but also without a line number. Something that can be understood at run-time, we're dealing with bytecode.

Nevertheless the situation is a bit similar to Java and you can actually quickly scope the above error using the call stack backtrace:

ReferenceError: Error #1069: Property property0 not found on Doc and there is no default value.
	at Doc/handleTimerResult()
	at flash.events::EventDispatcher/dispatchEventFunction()
	at flash.events::EventDispatcher/dispatchEvent()
	at flash.utils::Timer/tick()

so it runs up to handleTimerResult and then gets stuck. A quick dive into the (illustrative) code reveals that the trace is trying to access property0 during the while iteration at line 51.

package {
	import flash.display.MovieClip;
	import flash.utils.Timer;
	import flash.events.TimerEvent;

	public class Doc extends MovieClip {

		public var property1:uint;
		public var property2:uint;
		private var timer:Timer;

		public function Doc() {
			init();
		}

		private function init():void {
			trace("Doc Class initilized!");
			setProperties();
			initTimer();
		}

		private function setProperties():void {
			setProperty1();
			setProperty2();
		}

		public function setProperty2():void {
			property2 = randomOf100();
			trace("2nd property set to "+property2);
		}

		public function setProperty1():void {
			property1 = randomOf100();
			trace("1st property set to "+property1);
		}

		private function randomOf100():uint {
			return Math.random() * 100;
		}

		private function initTimer():void {
			timer = new Timer(1000, 5);
			timer.addEventListener(TimerEvent.TIMER, handleTimer);
			timer.addEventListener(TimerEvent.TIMER_COMPLETE, handleTimerResult);
			timer.start();
		}

		private function handleTimerResult(event:TimerEvent):void {
			var i:int = 3;
			while (i--) {
				trace("Property "+i+": "+this['property'+i]); // run-time error
			}
		}

		private function handleTimer(event:TimerEvent):void {
			setProperty1();
			setProperty2();
			trace("Properties reset!");
		}
	}
}

In most cases, if a class is structured reasonably, the above logic should deliver nicely. If this is not the case, in Flash IDE, instead of the regular Cmd/Ctrl + Enter "Test Movie" functionality, you also have "Debug Movie". It will launch standalone Flash Player and provide you with the line number in the backtrace:

ReferenceError: Error #1069: Property property0 not found on Doc and there is no default value.
	at Doc/handleTimerResult()[/Users/[...]/Doc.as:51]
	at flash.events::EventDispatcher/dispatchEventFunction()
	at flash.events::EventDispatcher/dispatchEvent()
	at flash.utils::Timer/tick()

In addition to the above there are loads of tools out there, e.g. MonsterDebugger. Check out Lee Brimelow's Debugging with MonsterDebugger video tutorial for more.

Related references: