第六章， confusion部分

JavaScript does not have a good mechanism for distinguishing between arrays
and objects. We can work around that deficiency by defining our own is_array
function:

var is_array = function (value) {
    return value &&
    typeof value === 'object' &&
    value.constructor === Array;
};
Unfortunately, it fails to identify arrays that were constructed in a
different window or frame.

最后一句话真心不懂，请教意思？
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FROM 122.235.241.*