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 if

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.