Javascript: top.name_of_variable does not produce try catch exceptions

This is more a curiosity, if you have the following code:

try {

podbridgeImg = “&win_id=” + top.win_id;
} catch(e) {
top.tmp_w = new Date();
top.win_id = top.tmp_w.getTime();
podbridgeImg = “&win_id=” + top.win_id;
}

You would expect it to fail when win_id is not set, and to run the catch part instead, but when you use top.variable_x, if variable_x is not defined, the code will not fail. In other words, try catch is not useful when you are dealing with variables on the top.variable_name domain