2.1.22 [ECMA-262/5] Section 126.96.36.199, Array.prototype.indexOf ( searchElement [ , fromIndex ] )
The specification states:
NOTE The indexOf 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 indexOf function can be applied successfully to a host object is implementation-dependent.
IE9 Mode, IE10 Mode, and IE11 Mode (All Versions)
The indexOf method can be applied to a host object; however, some host objects may reject such an application.