2.1.29 [ECMA-262/5] Section 18.104.22.168, Array.prototype.reduce ( callbackfn [ , initialValue ] )
The specification states:
NOTE The reduce function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method. Whether the reduce function can be applied successfully to a host object is implementation-dependent.
IE9 Mode, IE10 Mode, and IE11 Mode (All Versions)
The reduce method can be applied to a host object; however, some host objects may reject such an application.