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Comment by chrisdew | Hacker News Reader

    var undefined = "raspberry";

    function eatFruit (fruit) {
        if (fruit === undefined) {
            fruit = "strawberry";
        }
        ...
    }

    eatFruit("raspberry");

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I was intrigued and just tried this. It doesn't work (at least in the chrome console) -- "undefined" is a reserved keyword and while the interpreter doesn't complain about attempting to assign it the value of "raspberry," it appears to ignore the statement.

Apologies, you're right.

I'm sure this used to work, a few years ago... but I may be getting old.

But there's definitely still odd stuff here:

    $ node  
    > var bar = undefined = "foo";
    undefined
    > bar
    'foo'
    > undefined
    undefined
    >

"undefined" (and "NaN") are not a reserved keywords. They're just a properties of global object, with values of, respectively, primitive "undefined" and primitive "NaN" (which are not accessible directly).

In ES5 these properties are defined as [[Writable]]: false, which explains your behaviour.

You can however still create such variable in non-global scope - simply wrap that code in a function and it will work.

Not that I agree with the sentiment of the article (defaulting with || is usually okay!), but hopefully people reading these pieces, and worrying about such things, are already using strict mode.

you can also use...

    if (foo === void 0) ...

    //or
    var undef;
    ...
    if (foo === undef) ...

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