2.1.22 [ECMA-262/5] Section 188.8.131.52, Array.prototype.indexOf ( searchElement [ , fromIndex ] )
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.
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The indexOf method can be applied to a host object; however, some host objects may reject such an application.