indexOf(), includes(), lastIndexOf()

Syntax

arr.indexOf(value)
arr.includes(value)
arr.lastIndexOf(value)

Examples

const fruits = ["apple", "banana", "cherry", "banana"];

console.log(fruits.indexOf("banana"));   // 1 - first match
console.log(fruits.indexOf("grape"));    // -1 - not found
console.log(fruits.includes("cherry"));  // true
console.log(fruits.includes("grape"));   // false

const fruits = ["apple", "banana", "cherry", "banana"];
console.log(fruits.lastIndexOf("banana")); // 3 - the last match

const users = [{ name: "Alice" }, { name: "Bob" }];

console.log(users.includes({ name: "Alice" })); // false - different object reference!

// Use .some() with a custom comparison instead
console.log(users.some(u => u.name === "Alice")); // true

Best practices

• Use includes() when you only need a true/false answer - it is more readable than checking indexOf() !== -1
• Use indexOf() when you actually need the position of the match, such as for use with splice()
• Remember these methods use strict equality, so they cannot find objects/arrays by their content - use find() or some() with a custom comparison for that
• includes() correctly finds NaN (unlike indexOf(), which cannot) - a subtle but useful difference

At a glance

Purpose

Application logic and interaction

File extension

.js · .mjs

Runs in

Browsers and JavaScript runtimes

Usually used with

HTML, CSS and Web APIs

Specifications & further reading

• ECMAScript specification

Related JavaScript documentation