JavaScript String lastIndexOf() Method

What You’ll Learn

String.prototype.lastIndexOf() returns the last index of a substring, or -1 if it is missing (same API as MDN String.prototype.lastIndexOf()). Learn the optional from-position, case sensitivity, empty-string quirks, how it differs from indexOf(), five examples, and try-it labs.

Introduction

When a substring appears more than once and you care about the rightmost match — file extensions, the last slash in a path, the final keyword — use lastIndexOf(). It returns a zero-based index, or -1 when nothing matches.

The search is case-sensitive. An optional second argument limits the search to matches that start at or before that index. For the first match from the left, use indexOf().

This page is part of JavaScript String Methods. Related topics include indexOf() and includes().

Understanding the lastIndexOf() Method

lastIndexOf(searchString, position) looks for the last place the substring begins at an index ≤ position (default: end of the string).

• Indexes are zero-based — the first character is at 0.
• Missing substring — returns -1.
• Matching is case-sensitive and exact.
• position < 0 behaves like searching only at index 0.

📝 Syntax

General forms of String.prototype.lastIndexOf:

str.lastIndexOf(searchString)
str.lastIndexOf(searchString, position)

Parameters

• searchString — the substring to look for. Coerced to a string (undefined becomes "undefined").
• position (optional) — only consider matches that start at an index ≤ this value. Defaults to searching the whole string. If greater than str.length, the entire string is searched. If less than 0, only index 0 is considered.

Return value

A number — the last matching index, or -1. Special cases:

Common patterns

"canal".lastIndexOf("a");     // 3
"canal".lastIndexOf("a", 2);  // 1
"canal".lastIndexOf("x");     // -1

const path = "docs/guides/intro.md";
path.lastIndexOf(".");        // 16 — start of extension
path.slice(path.lastIndexOf(".") + 1);  // "md"

// First vs last:
"Brave, Brave New World".indexOf("Brave");      // 0
"Brave, Brave New World".lastIndexOf("Brave");  // 7

Examples Gallery

Examples follow MDN String.lastIndexOf() patterns. Use View Output or Try It Yourself for each case.

const paragraph = "I think Ruth's dog is cuter than your dog!";
const searchTerm = "dog";

`Index of the last "${searchTerm}" is ${paragraph.lastIndexOf(searchTerm)}`;
// 'Index of the last "dog" is 38'

Index of the last "dog" is 38

const anyString = "Brave, Brave New World";

anyString.indexOf("Brave");      // 0
anyString.lastIndexOf("Brave");  // 7

"canal".lastIndexOf("a");     // 3
"canal".lastIndexOf("a", 2);  // 1
"canal".lastIndexOf("a", 0);  // -1
"canal".lastIndexOf("x");     // -1

0
7
3
1
-1
-1

const s = "hello world hello";

s.lastIndexOf("world", 4);   // -1  (match at 6 is not ≤ 4)
s.lastIndexOf("hello", 99);  // 12  (last hello still ≤ 99)
s.lastIndexOf("hello", 0);   // 0   (only look at index 0)
s.lastIndexOf("hello", -5);  // 0   (negative → same as 0)

"canal".lastIndexOf("c", -5); // 0
"canal".lastIndexOf("c", 0);  // 0

-1
12
0
0
0
0

"Blue Whale, Killer Whale".lastIndexOf("Whale");  // 19
"Blue Whale, Killer Whale".lastIndexOf("blue");   // -1

"Blue Whale, Killer Whale"
  .toLowerCase()
  .lastIndexOf("blue");  // 0

19
-1
0

"canal".lastIndexOf("");     // 5  (length)
"canal".lastIndexOf("", 2);  // 2

const file = "archive.tar.gz";
const dot = file.lastIndexOf(".");
file.slice(dot + 1);         // "gz"
file.slice(0, dot);          // "archive.tar"

file.lastIndexOf("zip") !== -1;  // false
file.includes("tar");            // true (clearer boolean)

5
2
gz
archive.tar
false
true

🚀 Common Use Cases

• File extensions — last "." before slicing the suffix.
• Path segments — last "/" or "\\" to get the basename.
• Rightmost keyword — find the final occurrence of a token.
• Pair with indexOf — compare first vs last positions.
• Bounded search — pass position to ignore matches past a fence.
• Prefer includes for booleans — clearer intent for yes/no checks.

📝 Notes

• lastIndexOf() is case-sensitive.
• A miss returns -1 — never confuse that with index 0.
• The optional position is a right fence: match start must be ≤ that index.
• Negative position only considers index 0.
• Empty searchString has special return rules (see the table above).
• Omitting the needle (or passing undefined) searches for "undefined".
• Prefer includes() when you only need a boolean.

💡 Best Practices