Array.prototype.findIndex()

Basic Syntax

array.findIndex(callback(element, index, array))

// Returns the index of the first match, or -1

Same callback signature as find — but you get the position, not the element.

Find Index in an Array of Objects

const users = [
    { id: 10, name: 'Alice' },
    { id: 20, name: 'Bob' },
    { id: 30, name: 'Carol' }
];

const index = users.findIndex(u => u.id === 20);
// 1

if (index !== -1) {
    console.log('Found at index', index);
} else {
    console.log('Not found');
}

Always check !== -1 before using the index.

Update an Element by Index

const products = [
    { sku: 'A1', name: 'Widget' },
    { sku: 'B2', name: 'Gadget' },
    { sku: 'C3', name: 'Doohickey' }
];

// Find the index of the item to update
const idx = products.findIndex(p => p.sku === 'B2');

if (idx !== -1) {
    products[idx].name = 'Updated Gadget';
}
// products[1].name is now 'Updated Gadget'

Remove an Element by Index

const fruits = ['apple', 'banana', 'cherry', 'date'];

const idx = fruits.findIndex(f => f === 'cherry');

if (idx !== -1) {
    fruits.splice(idx, 1);
}
// fruits is now ['apple', 'banana', 'date']

The classic remove pattern: findIndex + splice. Safe, because we check for -1 first.

findIndex on Primitives

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

// Find index of specific value (use indexOf for this)
numbers.findIndex(n => n === 30); // 2
numbers.indexOf(30);              // 2

// Complex test — that's what findIndex is for
numbers.findIndex(n => n > 25 && n < 45); // 2 (matches 30)

Real-World: Check for Duplicates Before Adding

const cart = [
    { id: 1, name: 'Widget', qty: 1 },
    { id: 2, name: 'Gadget', qty: 3 }
];

function addToCart(item) {
    const existingIndex = cart.findIndex(c => c.id === item.id);

    if (existingIndex !== -1) {
        cart[existingIndex].qty += 1;
    } else {
        cart.push({ ...item, qty: 1 });
    }
}

addToCart({ id: 1, name: 'Widget' });
// cart[0].qty is now 2