How dangerous is it in JavaScript, really, to assume undefined is not overwritten?

Question

Every time anyone mentions testing against undefined, it\'s pointed out that undefined is not a keyword so it could be set to \"hello\", so you sho

Answer 1

No proper code will do such a thing. But you can never know what some wannabe-smart developer or a plugin/library/script you are using did. On the other side, it's extremely unlikely and modern browsers will not allow overwriting undefined at all, so if you are using such a browser for development you'll quickly notice if any code tries to overwrite it.

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And even though you did not ask for it - many people will probably find this question when looking for the more common "how to protect against redefined undefined" issue, so I'll answer that anyway:

There's a very good way to get a truly undefined undefined no matter how old the browser is:

(function(undefined) {
    // your code where undefined is undefined
})();

This works because an argument that is not specified is always undefined. You can also do it with a function that accepts some real arguments, e.g. like this when you are using jQuery. It's usually a good idea to ensure a sane environment in this way:

(function($, window, undefined) {
    // your code where undefined is undefined
})(jQuery, this);

Then you can be sure that inside that anonymous function the following things are true:

• $ === jQuery
• window === [the global object]
• undefined === [undefined].

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However, note that sometimes typeof x === 'undefined' is actually necessary: If the variable x has never been set to a value (contrary to being set to undefined), reading x in a different way such as if(x === undefined) will throw an error. This does not apply to object properties though, so if you know that y is always an object, if(y.x === undefined) is perfectly safe.