How to find the index of an element in JavaScript

The findIndex() method returns the index of the first element in an array that satisfies a provided testing function. If no element passes the test, it returns -1. This is particularly useful when you need to find the position of an element that matches specific criteria.

const numbers = [10, 20, 30, 40, 50];

const index = numbers.findIndex(num => num > 25);
console.log(index);
// 2 (30 is the first number greater than 25)

const notFound = numbers.findIndex(num => num > 100);
console.log(notFound);
// -1 (no numbers are greater than 100)

It's especially useful when working with arrays of objects:

const users = [
{ id: 1, name: 'John', age: 25 },
{ id: 2, name: 'Jane', age: 30 },
{ id: 3, name: 'Bob', age: 35 }
];

const janeIndex = users.findIndex(user => user.name === 'Jane');
console.log(janeIndex);
// 1

const underageIndex = users.findIndex(user => user.age < 18);
console.log(underageIndex);
// -1

Unlike the indexOf() method which only checks for exact matches, findIndex() allows you to define custom matching logic:

const fruits = ['apple', 'banana', 'orange', 'mango'];

// indexOf can only find exact matches
console.log(fruits.indexOf('banana'));
// 1

// findIndex can use custom logic
console.log(fruits.findIndex(fruit => fruit.length > 5));
// 1 (banana is the first fruit with length > 5)