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# Array.prototype.findIndex

The **`findIndex()`** method returns the index of the first element in an array that satisfies the provided testing function.
If no elements satisfy the testing function, -1 is returned.

See also the [`find()`](./find.mdx) method, which returns the first element that satisfies the testing function (rather than its index).

## Syntax

```js
// Arrow function
findIndex((element) => { /* … */ })
findIndex((element, index) => { /* … */ })
findIndex((element, index, array) => { /* … */ })

// Callback function
findIndex(callbackFn)
findIndex(callbackFn, thisArg)

// Inline callback function
findIndex(function (element) { /* … */ })
findIndex(function (element, index) { /* … */ })
findIndex(function (element, index, array) { /* … */ })
findIndex(function (element, index, array) { /* … */ }, thisArg)
```

### Parameters

- `callbackFn`

  - : A function to execute for each element in the array. It should return a [truthy](https://developer.mozilla.org/docs/Glossary/Truthy) value to indicate a matching element has been found.

    The function is called with the following arguments:

    - `element`
      - : The current element being processed in the array.
    - `index`
      - : The index of the current element being processed in the array.
    - `array`
      - : The array `findIndex()` was called upon.

- `thisArg` _**optional**_
  - : A value to use as `this` when executing `callbackFn`.

### Return value

The index of the first element in the array that passes the test. Otherwise, `-1`.

## Description

The `findIndex()` is an iterative method. It calls a provided `callbackFn` function once for each element in an array in ascending-index order, until `callbackFn` returns a [truthy](https://developer.mozilla.org/docs/Glossary/Truthy) value. `findIndex()` then returns the index of that element and stops iterating through the array. If `callbackFn` never returns a truthy value, `findIndex()` returns `-1`.

`callbackFn` is invoked for _every_ index of the array, not just those with assigned values. Empty slots in sparse arrays behave the same as `undefined`.

`findIndex()` does not mutate the array on which it is called, but the function provided as `callbackFn` can. Note, however, that the length of the array is saved _before_ the first invocation of `callbackFn`. Therefore:

- `callbackFn` will not visit any elements added beyond the array's initial length when the call to `findIndex()` began.
- Changes to already-visited indexes do not cause `callbackFn` to be invoked on them again.
- If an existing, yet-unvisited element of the array is changed by `callbackFn`, its value passed to the `callbackFn` will be the value at the time that element gets visited. [Deleted](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Operators/delete) elements are visited as if they were `undefined`.

The `findIndex()` method is generic. It only expects the `this` value to have a `length` property and integer-keyed properties.
