Array.prototype.findIndex()

findIndex(callbackFn, thisArg)

Returns: number · Added in vES6 · Updated March 13, 2026 · Array Methods

array find search index

findIndex() iterates through an array and returns the index of the first element that passes the test function. If no element passes, it returns -1. Unlike find() which returns the element itself, findIndex() gives you the position in the array.

Syntax

arr.findIndex(callbackFn, thisArg)

Parameters

Parameter	Type	Default	Description
`callbackFn`	`function`	—	Function to test each element
`element`	`any`	—	Current element being processed
`index`	`number`	—	Index of current element
`array`	`array`	—	Array that findIndex was called on
`thisArg`	`object`	`undefined`	Value to use as `this` when executing callbackFn

The callback function should return true for elements you want to find.

Examples

Finding a user position

const users = [
  { name: "Alice", role: "admin" },
  { name: "Bob", role: "editor" },
  { name: "Charlie", role: "admin" }
];

const firstAdmin = users.findIndex(user => user.role === "admin");
console.log(firstAdmin);
// 0 (Alice is at index 0)

// If not found, returns -1
const notFound = users.findIndex(user => user.role === "owner");
console.log(notFound);
// -1

Finding the first even number

const numbers = [1, 3, 5, 8, 9, 10];

const firstEven = numbers.findIndex(n => n % 2 === 0);
console.log(firstEven);
// 3 (the number 8 at index 3)

Using thisArg

const threshold = { min: 10 };

const index = [5, 8, 12, 3].findIndex(function(item) {
  return item > this.min;
}, threshold);

console.log(index);
// 2 (12 is the first value greater than 10)

Common Patterns

Finding and updating an element

const items = [
  { id: 1, name: "Widget", price: 29.99 },
  { id: 2, name: "Gadget", price: 49.99 }
];

// Find the index, then update
const idx = items.findIndex(item => item.id === 2);
if (idx !== -1) {
  items[idx] = { ...items[idx], price: 39.99 };
}
console.log(items[1].price);
// 39.99

Chaining with other array methods

const products = [
  { name: "Laptop", inStock: false, price: 999 },
  { name: "Mouse", inStock: true, price: 29 },
  { name: "Keyboard", inStock: true, price: 79 }
];

// Find first in-stock item, then get its price
const firstInStockIdx = products.findIndex(p => p.inStock);
const price = products[firstInStockIdx]?.price;

console.log(price);
// 29

When to Use

When you need the position of an element rather than the element itself
When you will use the index for further operations (splicing, updating)
When searching large arrays where finding the position is more efficient than searching twice

When Not to Use

Use find() if you need the element, not the index
Use indexOf() for simple value equality checks (faster)
Use includes() if you only need to know whether an element exists