JavaScript lastIndexOf method

last modified April 4, 2025

In this article we show how to find elements using the lastIndexOf method in JavaScript for both arrays and strings.

The lastIndexOf method

The lastIndexOf method returns the last index at which a given element can be found in an array or string. For arrays, it searches backwards from the end. For strings, it searches backwards from the specified index.

The method returns -1 if the element is not found. For arrays, it uses strict equality (===) for comparison. For strings, it performs a case-sensitive search. The method is available on both Array and String prototypes in JavaScript.

The lastIndexOf method can take an optional second parameter to specify the starting index for the search. Negative values count back from the end of the array or string. This makes it useful for finding the last occurrence of an element.

Basic array lastIndexOf example

The following example demonstrates the basic usage of lastIndexOf with arrays.

main.js

const fruits = [‘apple’, ‘banana’, ‘orange’, ‘banana’, ‘kiwi’]; const lastBanana = fruits.lastIndexOf(‘banana’);

console.log(Last banana at index: ${lastBanana}); console.log(Mango not found: ${fruits.lastIndexOf('mango')});

We create an array of fruits and search for the last occurrence of ‘banana’. The method returns 3, which is the index of the last ‘banana’. When searching for ‘mango’, it returns -1 as it doesn’t exist in the array.

$ node main.js Last banana at index: 3 Mango not found: -1

String lastIndexOf example

The lastIndexOf method works similarly with strings.

main.js

const quote = “To be, or not to be, that is the question.”; const lastBe = quote.lastIndexOf(“be”);

console.log(Last 'be' at index: ${lastBe}); console.log(Case sensitive search: ${quote.lastIndexOf("Be")});

We search a string for the last occurrence of “be”. The method returns the index where it’s found. Note that the search is case-sensitive, so searching for “Be” returns -1 as it doesn’t match exactly.

$ node main.js Last ‘be’ at index: 17 Case sensitive search: -1

Using the fromIndex parameter

The lastIndexOf method accepts an optional second parameter to specify where to start searching.

main.js

const numbers = [2, 5, 9, 2, 5, 9, 2]; const search1 = numbers.lastIndexOf(2); const search2 = numbers.lastIndexOf(2, 3); const search3 = numbers.lastIndexOf(2, -3);

console.log(Search1: ${search1}); console.log(Search2: ${search2}); console.log(Search3: ${search3});

We demonstrate different searches with the fromIndex parameter. The first search finds the last 2 at index 6. The second starts at index 3 and finds 2 at index 3. The third uses a negative index, counting back from the end.

$ node main.js Search1: 6 Search2: 3 Search3: 0

Finding all occurrences with lastIndexOf

We can use lastIndexOf in a loop to find all occurrences of an element.

main.js

const data = [1, 2, 3, 4, 2, 5, 2, 6]; const target = 2; let indices = []; let currentIndex = data.lastIndexOf(target);

while (currentIndex !== -1) { indices.push(currentIndex); currentIndex = data.lastIndexOf(target, currentIndex - 1); }

console.log(All indices of ${target}:, indices);

This code finds all indices where the number 2 appears in the array. We start with the last occurrence and work backwards, updating the search position each time. The loop continues until no more occurrences are found.

$ node main.js All indices of 2: [ 6, 4, 1 ]

Complex object search with lastIndexOf

The lastIndexOf method uses strict equality, which affects how it works with objects.

main.js

const obj1 = { id: 1 }; const obj2 = { id: 2 }; const obj3 = { id: 1 }; const objects = [obj1, obj2, obj3, obj1];

console.log(Last obj1: ${objects.lastIndexOf(obj1)}); console.log(Last obj3: ${objects.lastIndexOf(obj3)}); console.log(Same content but different object: ${objects.lastIndexOf({ id: 1 })});

We demonstrate how lastIndexOf works with objects. It only finds objects that reference the exact same object in memory. Even though obj1 and obj3 have the same content, they are different objects. A new object with the same content is not found at all.

$ node main.js Last obj1: 3 Last obj3: 2 Same content but different object: -1

Source

Array lastIndexOf - language reference

In this article we have demonstrated how to use the lastIndexOf() method to find elements in arrays and strings in JavaScript.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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