Ultimate equivalence of PHP's isset

[lwc]

This is also served to declare default values in functions.

Seemingly, the solution is:

if (myvar == undefined)

Alas,

Some really old browsers don’t support undefined.

A solution is given for this:

add a global def (with no value) for undefined, e.g.
var undefined;

But I'd like to ask fellow posters here if it's really the right solutions.

[Fang]

Use undefined. I doubt there are any IE5 browsers that have never been updated.