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Comment by 19h | Hacker News Reader

It’s also wrong because the representation wasn’t exactly always deterministic in 2013 across all js engines. The right way to check for an array is value.constructor === Array, which also doesn’t really make any sense because an array in JS really is just an object with a length key and respective keys names 0…n.

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I've been using JS on and off in various incarnations, and this sort of blows my mind. First because I don't think I've ever seen that approach used anywhere, and second because it would probably take me upwards of an hour (possibly a day) to work out whether that would actually work.

The constructor check would unfortunately not work for cross-frame arrays. Each frame has its own global object, with its own Array constructor. So:

    var iframe = document.createElement('frame');
    document.body.appendChild(iframe);

    var value = iframe.contentWindow.Array();
    value.constructor === Array // false
    value instanceof Array // false
    value instanceof iframe.contentWindow.Array // true

Stringifying the constructor works even for cross-frame values:

    value.constructor.toString() === Array.toString() // true

These days though, `Array.isArray` is the right thing to do, available since roughly 2010.

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