What makes Firebug/Chrome console treat a custom object as an array?

Question

When I am developing in jQuery, I frequently find myself typing selectors into the Chrome/Firebug console and seeing what they give me. They are always nicely formatted as

Answer 1

This is what Firebug's isArray method does: (from the Firebug source)

if (!obj)
    return false;
else if (isIE && !isFunction(obj) && typeof obj == "object" && isFinite(obj.length) && obj.nodeType != 8)
    return true;
else if (isFinite(obj.length) && isFunction(obj.splice))
    return true;
else if (isFinite(obj.length) && isFunction(obj.callee)) // arguments
    return true;
else if (instanceOf(obj, "HTMLCollection"))
    return true;
else if (instanceOf(obj, "NodeList"))
    return true;
else
    return false;

Of course, none of these checks ensures that the object is a true JavaScript array, but they do a reasonable job of guessing whether an object is a pseudo-array, which in turn gives you a convenient array-like representation for debugging.

Chrome may or may not use these same checks, and the new Web Console in Firefox 4 doesn't recognize anything other than true arrays as arrays.