There is not much difference here but for debugging this is useful (and it is more technically correct)
Basically, if you have an object
var foo = {
'x': 1,
'y': 2,
'z': 3
};
and you wish to remove the 'y' property, you can say:
foo.y = null;
...and that will be fine, the value is gone.
If, however, you wish to totally eliminate it from your object, the proper procedure is:
delete foo.y;
The difference is that if some day you choose to iterate over the keys in foo, nullifying 'y' will not remove the key 'y'. Calling delete will.
In the patch attached, I make this change so that later, when iterating through in unloadCache(), the keys will not show up if they have been totally removed.
OpenLayers
