Question

filter    Creates a new array with all elements that pass
          the test implemented by the provided function.

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>>> 'abc'.filter(function(e){return e!='b'})
TypeError: "abc".filter is not a function

>>> Array.prototype.filter.call('abc', function(e){return e!='b'})
["a", "c"]

>>> jQuery.isArray('abc')
false

Answer 1

All array methods are generic; i.e., they work on any object with numeric properties plus a length property. See the specification for Array.prototype.filter, for example: note that the algorithm is phrased entirely in terms of things like

Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".

or

Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.

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Array.filter is a Firefox-specific extension, BTW. The cross-browser version is Array.prototype.filter.call.