Does it mutate 😱

Array.prototype.lastIndexOf

No mutation

Description

The lastIndexOf() method returns the last index at which a given element can be found in the array, or -1 if it is not present. The array is searched backwards, starting at fromIndex.

Array.prototype.lastIndexOf ( searchElement [ , fromIndex ] )

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/lastIndexOf

Example

var animals = ['Dodo', 'Tiger', 'Penguin', 'Dodo'];

console.log(animals.lastIndexOf('Dodo'));
// expected output: 3

console.log(animals.lastIndexOf('Tiger'));
// expected output: 1