Javascript: Control over undefined?

Maybe somehow extend try&catch or errors handling to throw errors on undefined catch, only in debug mode(flag on)?

Because it will lessen my code and increase readability.

if (typeof foo != 'undefined') {
 /* Do something with foo */
}

Instead of using it right away.

Same with concatenating undefined variable to a string if there any solution.

'bar'+ ((typeof foo != 'undefined') ? foo : '') +'bar'

Instead of using it right away

'bar'+ foo +'bar'

Thanks :)

Update, some examples:

(function(o){
var s = {
    foo:''
}
$.extend(s,o);

console.log('testing'+ s.foo +'testing');
})();

(function(o){
console.log('testing'+ o.foo +'testing');
})({foo:''});

(function(o){
console.log('testing'+ (typeof o.foo != 'undefined' ? o.foo : '') +'testing');
})();
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(function(o){
console.log('testing'+ o.foo +'testing');
})();

---

Your example:

(function(o){
    var s = {
        foo:''
    }
    $.extend(s,o);

    console.log('testing'+ s.foo +'testing');
})();

is the best way, in my opinion, to have a default value for an argument.

Other than that, I'm not quite sure what this question is asking. There is no way in JavaScript to extend the behavior of operators or keywords, such as try..catch. By extension, the default value of undefined can't be altered either. The best you can do is the above example.