What is the correct way to check if a global variable exists?

Question

JSLint is not passing this as a valid code:

/* global someVar: false */
if (typeof someVar === \"undefined\") {
    var someVar = \"hi!\";
}

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Answer 1

I think this is actually a problem with JSLint. It will issue the following error:

Unexpected 'typeof'. Compare directly with 'undefined'.

I believe this is bad advice. In JavaScript, undefined is a global variable that is, usually, undefined. But some browsers allow scripts to modify it, like this: window.undefined = 'defined'. If this is the case, comparing directly with undefined can lead to unexpected results. Fortunately, current ECMA 5 compliant browsers do not allow assignments to undefined (and will throw an exception in strict mode).

I prefer typeof someVar === "undefined", as you posted, or someVar in window as Susei suggested.