Testing for undefined and null child objects in ActionsScript/Flex

Question

Hi Everyone,

I use this pattern to test for undefined and null values in ActionScript/Flex :

if(obj) {
    execute()
}

Unfortunately, a ReferenceError is always thrown when I use the pattern to test for child objects :

if(obj.child) {
    execute()
}

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

Why does testing for child objects with if statements throw a ReferenceError?

Thanks!

Answer 1

This happens when obj is a strongly typed object but it doesn't have a child field.

You can test to see if a field exists on any object using the in operator:

if ("foo" in obj && obj.foo)
    execute();

I've also written a utility function to make this process easier:

function getattr(obj:Object, field:*, dflt:*=undefined):* {
    if (field in obj && obj[field])
        return obj[field];
    return dflt;
}

Answer 2

You're getting this error because the obj's type does not have the child property in it. You need to do something like this:

if((obj) && (obj.hasOwnProperty('child') && (obj.child)){
 execute()
}

More info on the hasOwnProperty method in the Object class: http://livedocs.adobe.com/flex/3/langref/Object.html#hasOwnProperty%28%29

Answer 3

You can avoid reference errors by using array notation:

if([obj.name][child.name]){
execute();
}

The important thing to realize is that simply avoiding the error can cause issues down the track - debugging will be harder in larger applications.

Of course, the ideal approach is to completely avoid the situation by using validator functions to ensure you have the right data, instead of testing for null when the data is required. :)