npm package: @stdlib/utils-index-of
Description: Return the first index at which a given element can be found.
Author: The Stdlib Authors
Latest version: 0.2.3

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</details>

# indexOf

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> Return the first index at which a given element can be found.

<section class="installation">

## Installation

```bash
npm install @stdlib/utils-index-of
```

</section>

<section class="usage">

## Usage

```javascript
var indexOf = require( '@stdlib/utils-index-of' );
```

#### indexOf( arr, searchElement\[, fromIndex] )

Returns the first index at which a given element can be found.

```javascript
var arr = [ 4, 3, 2, 1 ];

var idx = indexOf( arr, 3 );
// returns 1
```

If a `searchElement` is **not** present in an input `array`, the function returns `-1`.

```javascript
var arr = [ 4, 3, 2, 1 ];

var idx = indexOf( arr, 5 );
// returns -1
```

By default, the implementation searches an input `array` starting from the first element. To start searching from a different element, specify a `fromIndex`.

```javascript
var arr = [ 1, 2, 3, 4, 5, 2, 6 ];

var idx = indexOf( arr, 2, 3 );
// returns 5
```

If a `fromIndex` exceeds the input `array` length, the function returns `-1`.

```javascript
var arr = [ 1, 2, 3, 4, 2, 5 ];

var idx = indexOf( arr, 2, 10 );
// returns -1
```

If a `fromIndex` is less than `0`, the starting index is determined relative to the last index (with the last index being equivalent to `fromIndex = -1`).

```javascript
var arr = [ 1, 2, 3, 4, 5, 2, 6, 2 ];

var idx = indexOf( arr, 2, -4 );
// returns 5

idx = indexOf( arr, 2, -1 );
// returns 7
```

If `fromIndex` is less than `0` **and** its absolute value exceeds the input `array` length, the function searches the entire input `array`.

```javascript
var arr = [ 1, 2, 3, 4, 5, 2, 6 ];

var idx = indexOf( arr, 2, -10 );
// returns 1
```

The first argument is not limited to `arrays`, but may be any [array-like][@stdlib/assert/is-array-like] `object`.

```javascript
var str = 'bebop';

var idx = indexOf( str, 'o' );
// returns 3
```

</section>

<!-- /.usage -->

<section class="notes">

## Notes

-   Search is performed using **strict equality** comparison. Thus,

    ```javascript
    var arr = [ 1, [ 1, 2, 3 ], 3 ];

    var idx = indexOf( arr, [ 1, 2, 3 ] );
    // returns -1
    ```

-   This implementation is **not** [ECMAScript Standard][ecma-262] compliant. Notably, the [standard][ecma-262] specifies that an `array` be searched by calling `hasOwnProperty` (thus, for most cases, incurring a performance penalty), and the [standard][ecma-262] does **not** accommodate a `searchElement` equal to `NaN`. In this implementation, the following is possible:

    ```javascript
    // Locate the first element which is NaN...
    var arr = [ 1, NaN, 2, NaN ];

    var idx = indexOf( arr, NaN );
    // returns 1

    // Prototype properties may be searched as well...
    function Obj() {
        this[ 0 ] = 'beep';
        this[ 1 ] = 'boop';
        this[ 2 ] = 'woot';
        this[ 3 ] = 'bap';
        this.length = 4;
        return this;
    }
    Obj.prototype[ 2 ] = 'bop';

    var obj = new Obj();

    idx = indexOf( obj, 'bop' );
    // returns -1

    delete obj[ 2 ];

    idx = indexOf( obj, 'bop' );
    // returns 2
    ```

</section>

<!-- /.notes -->

<section class="examples">

## Examples

<!-- eslint no-undef: "error" -->

```javascript
var indexOf = require( '@stdlib/utils-index-of' );

var arr;
var obj;
var str;
var idx;
var i;

// Arrays...
arr = new Array( 10 );
for ( i = 0; i < arr.length; i++ ) {
    arr[ i ] = i * 10;
}
idx = indexOf( arr, 40 );

console.log( idx );
// => 4

// Array-like objects...
obj = {
    '0': 'beep',
    '1': 'boop',
    '2': 'bap',
    '3': 'bop',
    'length': 4
};

idx = indexOf( obj, 'bap' );

console.log( idx );
// => 2

// Strings...
str = 'beepboopbop';

idx = indexOf( str, 'o' );

console.log( idx );
// => 5
```

</section>

<!-- /.examples -->

<!-- Section for related `stdlib` packages. Do not manually edit this section, as it is automatically populated. -->

<section class="related">

</section>

<!-- /.related -->

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<section class="main-repo" >

* * *

## Notice

This package is part of [stdlib][stdlib], a standard library for JavaScript and Node.js, with an emphasis on numerical and scientific computing. The library provides a collection of robust, high performance libraries for mathematics, statistics, streams, utilities, and more.

For more information on the project, filing bug reports and feature requests, and guidance on how to develop [stdlib][stdlib], see the main project [repository][stdlib].

#### Community

[![Chat][chat-image]][chat-url]

---

## License

See [LICENSE][stdlib-license].


## Copyright

Copyright &copy; 2016-2026. The Stdlib [Authors][stdlib-authors].

</section>

<!-- /.stdlib -->

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[stdlib]: https://github.com/stdlib-js/stdlib

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[umd]: https://github.com/umdjs/umd
[es-module]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Modules

[deno-url]: https://github.com/stdlib-js/utils-index-of/tree/deno
[deno-readme]: https://github.com/stdlib-js/utils-index-of/blob/deno/README.md
[umd-url]: https://github.com/stdlib-js/utils-index-of/tree/umd
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[esm-url]: https://github.com/stdlib-js/utils-index-of/tree/esm
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[branches-url]: https://github.com/stdlib-js/utils-index-of/blob/main/branches.md

[stdlib-license]: https://raw.githubusercontent.com/stdlib-js/utils-index-of/main/LICENSE

[ecma-262]: http://www.ecma-international.org/ecma-262/6.0/#sec-array.prototype.indexof

[@stdlib/assert/is-array-like]: https://www.npmjs.com/package/@stdlib/assert-is-array-like

</section>

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