(not implemented) Finds the last index within a collection that passes a test, searching from the right.
At time of writing, no collection implements this method, but it would be be a
logical addition triangulated from findIndex (an ECMA6 Proposal) and
lastIndexOf.
(not implemented) Finds the first value within a collection that passes a test. Finds the first index within a collection that passes a test.
Returns the last position of a value, or -1 if the value is not found.
(not implemented) Finds the last value within a collection that passes a test, searching from the right.
Finds the last equivalent value, searching from the right.