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 from the specified fromIndex.
Example 1: Finding the Last Occurrence
// An array with a repeated element.
const items = ['pen', 'pencil', 'pen', 'eraser'];
// The index of the last occurrence of 'pen' is found.
const lastPenIndex = items.lastIndexOf('pen');
// The last index is logged.
console.log(lastPenIndex); // Output: 2
Explanation: This example demonstrates the basic functionality of lastIndexOf() to find the index of the last occurrence of an element.
Example 2: Element Not Found
// An array of fruits.
const fruits = ['apple', 'banana', 'orange'];
// The last index of 'grape' is searched, which is not in the array.
const grapeIndex = fruits.lastIndexOf('grape');
// The result is logged to the console.
console.log(grapeIndex); // Output: -1
Explanation: Similar to indexOf(), lastIndexOf() returns -1 if the element is not found in the array.
Example 3: Searching Backwards from an Index
// An array with multiple occurrences of a number.
const numbers = [2, 5, 9, 2, 8, 2];
// The last index of 2 is searched for, starting the search backwards from index 4.
const indexBeforeLast = numbers.lastIndexOf(2, 4);
// The found index is logged.
console.log(indexBeforeLast); // Output: 3
Explanation: The optional fromIndex argument in lastIndexOf() specifies the index at which to start searching backwards.
Example 4: Finding the Last Index of a String
// An array of strings.
const logs = ['info', 'error', 'info', 'warning', 'info'];
// The last index of 'info' is found.
const lastInfoIndex = logs.lastIndexOf('info');
// The index is logged to the console.
console.log(lastInfoIndex); // Output: 4
Explanation: lastIndexOf() works effectively with string elements to find the last occurrence within the array.
Example 5: Case-Sensitive Last Index Search
// An array of strings with varied casing.
const words = ['Test', 'test', 'TEST', 'test'];
// The last index of 'test' (lowercase) is found.
const lastTestIndex = words.lastIndexOf('test');
// The index is logged.
console.log(lastTestIndex); // Output: 3
Explanation: Just like indexOf(), lastIndexOf() is case-sensitive, distinguishing between elements with different capitalization.
Example 6: Using a Negative fromIndex
// An array of characters.
const chars = ['a', 'b', 'c', 'd', 'a', 'e'];
// Search for 'a' starting backwards from the 2nd to last element.
const lastAFromNegativeIndex = chars.lastIndexOf('a', -2);
// The index is logged.
console.log(lastAFromNegativeIndex); // Output: 4
Explanation: A negative fromIndex is treated as an offset from the end of the array. The search still proceeds backwards from that point.
Example 7: Comparing indexOf and lastIndexOf
// An array with a repeated value.
const data = [1, 5, 3, 5, 9];
const firstFive = data.indexOf(5);
const lastFive = data.lastIndexOf(5);
// The first and last indices of 5 are logged.
console.log(`First occurrence of 5: ${firstFive}`); // Output: First occurrence of 5: 1
console.log(`Last occurrence of 5: ${lastFive}`); // Output: Last occurrence of 5: 3
Explanation: This example clearly illustrates the difference between indexOf() and lastIndexOf(), showing how they find the first and last occurrences of an element, respectively.