Array.prototype.lastIndexof
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
.
Syntax
arr.lastIndexOf(searchElement, fromIndex = arr.length  1])
Parameters

searchElement
 Element to locate in the array.

fromIndex
 Optional . The index at which to start searching backwards. Defaults to the array’s length minus one, i.e. the whole array will be searched. If the index is greater than or equal to the length of the array, the whole array will be searched. If negative, it is taken as the offset from the end of the array. Note that even when the index is negative, the array is still searched from back to front. If the calculated index is less than 0, 1 is returned, i.e. the array will not be searched.
Returns
The index of the last occurrence of searchElement
in the array, or 1 if searchElement
is not found.
Description
lastIndexOf
compares searchElement
to elements of the Array using strict equality (the same method used by the ===, or tripleequals, operator).
Remarks
The search occurs in descending index order (last member first). To search in ascending order, use the indexOf
method.
The optional fromIndex
argument specifies the array index at which to begin the search. If fromIndex
is greater than or equal to the array length, the whole array is searched. If fromIndex
is negative, the search starts at the array length plus fromIndex
. If the computed index is less than 0, 1 is returned.
Examples
var array = [2, 5, 9, 2];
array.lastIndexOf(2); // 3
array.lastIndexOf(7); // 1
array.lastIndexOf(2, 3); // 3
array.lastIndexOf(2, 2); // 0
array.lastIndexOf(2, 2); // 0
array.lastIndexOf(2, 1); // 3
// Create an array.
var ar = ["ab", "cd", "ef", "ab", "cd"];
// Determine the first location, in descending order, of "cd".
document.write(ar.lastIndexOf("cd") + "<br/>");
// Output: 4
// Find "cd" in descending order, starting at index 2.
document.write(ar.lastIndexOf("cd", 2) + "<br/>");
// Output: 1
// Search for "gh" (which is not found).
document.write(ar.lastIndexOf("gh")+ "<br/>");
// Output: 1
// Find "ab" with a fromIndex argument of 3.
// The search in descending order starts at index 3,
// which is the array length minus 2.
document.write(ar.lastIndexOf("ab", 3) + "<br/>");
// Output: 0