Flex 3: How can I see if a variable exists

Question

I'm getting the "Error #1009: Cannot access a property or method of a null object reference." error on my application. Is there a function I can use to detect this before it causes an error... maybe something like:

isValid(variableName);

I know there's one, because i've used it before, but i can't remember what it is right now.

Any help would be greatly appreciated.

http://stackoverflow.com/questions/296861/test-if-an-object-is-defined-in-actionscript Same question right there — az4dan, Jul 15 '11 at 20:38
I voted to close due to the duplicate question. http://stackoverflow.com/questions/296861/test-if-an-object-is-defined-in-actionscript — JeffryHouser, Jul 15 '11 at 20:45
Nope... this isn't the same. There's different way. Ugh, I'll keep on looking for it. I'll post it when i find it. — Brds, Jul 15 '11 at 21:10

score 2 · Accepted Answer · answered Jul 15 '11 at 22:40

2

My guess: hasOwnProperty

answered Jul 15 '11 at 22:40

okulin

121
4

score 2 · Answer 2 · answered Jul 16 '11 at 13:11

Simply put, a null object maps to a Boolean false. suppose:

var x:ArrayCollection; //uninitialised
if(x) {
    Alert.show("X");
} else {
    Alert.show("NOT X");
}

Above code will show an alert saying NOT X because a null variable maps to false

However, if you want to check whether an object has a property with a particular name, try

var o:MyObject=new MyObject();
if(o.hasOwnProperty("something")) {
    Alert.show(o.something);
} else {
    Alert.show("Something undefined");
}

now if there is a property called "something" on o, EVEN IF ITS VALUE IS null, it will go into if()... otherwise it will go into else.

Jason Towne · Answer 3 · 2011-07-18T15:15:38.537

1

A simple if statement will work.

if (myVariable)
{
  //do something
}

UPDATE: After looking at the code that's causing the error, my guess is that either wholeProject[j] is null or wholeProject[j].wholePosition is null. Try something like this:

if (wholeProject[j] && wholeProject[j].wholePosition)
{
  for (var k:int = 0; k < wholeProject[j].wholePosition.length; k++)
}

edited Jul 18 '11 at 15:15

answered Jul 15 '11 at 20:44

Jason Towne

8,014
5
54
69

@Brds Can you post the line of code that's causing the error? – Jason Towne Jul 15 '11 at 22:13
for (var k:int = 0; k < wholeProject[j].wholePosition.length; k++) – Brds Jul 18 '11 at 13:59
@Brds I just updated my answer based on the code you posted. Let me know if it helps. – Jason Towne Jul 18 '11 at 15:16

score 1 · Answer 4 · answered Jul 15 '11 at 21:50

It's actually quite simple. Use a try/catch construct with the (err:) For example, I use this to surround parsing code that can generate errors. "Error" means any error.

try { relation.parseObject(XMLObject["relation"],source); } 
catch (err:Error) {tr.output(mN + "bad relation " + err)};

You would do this:

try {
newvalue = variableName;
}
catch (error:ReferenceError) { <do something> }

Flex 3: How can I see if a variable exists

4 Answers4