Return the index of the first element which equals a provided search element.
npm install @stdlib/array-base-index-ofWe believe in a future in which the web is a preferred environment for numerical computation. To help realize this future, we've built stdlib. stdlib is a standard library, with an emphasis on numerical and scientific computation, written in JavaScript (and C) for execution in browsers and in Node.js. The library is fully decomposable, being architected in such a way that you can swap out and mix and match APIs and functionality to cater to your exact preferences and use cases. When you use stdlib, you can be absolutely certain that you are using the most thorough, rigorous, well-written, studied, documented, tested, measured, and high-quality code out there. To join us in bringing numerical computing to the web, get started by checking us out on GitHub, and please consider financially supporting stdlib. We greatly appreciate your continued support!
About stdlib...
[![NPM version][npm-image]][npm-url] [![Build Status][test-image]][test-url] [![Coverage Status][coverage-image]][coverage-url]
> Return the index of the first element which equals a provided search element.
``bash`
npm install @stdlib/array-base-index-of
`javascript`
var indexOf = require( '@stdlib/array-base-index-of' );
#### indexOf( x, searchElement, fromIndex )
Returns the index of the first element which equals a provided search element.
`javascript
var x = [ 1, 2, 3, 4, 5, 6 ];
var idx = indexOf( x, 3, 0 );
// returns 2
`
If the function is unable to find an element which equals a provided search element, the function returns -1.
`javascript
var x = [ 1, 2, 3, 4, 5, 6 ];
var idx = indexOf( x, 7, 0 );
// returns -1
`
To begin searching from specific index, provide a non-zero fromIndex argument.
`javascript
var x = [ 1, 1, 2, 1, 2, 3, 3 ];
var idx = indexOf( x, 2, 3 );
// returns 4
`
If fromIndex is less than zero, the starting index is resolved relative to the last array element, with the last array element corresponding to fromIndex = -1.
`javascript
var x = [ 1, 1, 2, 1, 2, 3, 3 ];
var idx = indexOf( x, 2, -4 );
// returns 4
`
- If provided an array-like object having an indexOf method, the function defers execution to that method and assumes that the method API has the following signature:
`text`
x.indexOf( searchElement, fromIndex )
- If provided an array-like object without an indexOf method, the function performs a linear scan and returns immediately upon finding a match.
- When searching for a search element, the function checks for equality using the strict equality operator ===. As a consequence, NaN values are considered distinct, and -0 and +0 are considered the same.
`javascript
var indexOf = require( '@stdlib/array-base-index-of' );
var x = [ 'foo', 'bar', 'beep', 'boop', 'foo', 'bar' ];
var idx = indexOf( x, 'beep', 0 );
// returns 2
idx = indexOf( x, 'bop', 0 );
// returns -1
idx = indexOf( x, 'foo', 1 );
// returns 4
idx = indexOf( x, 'foo', -4 );
// returns 4
idx = indexOf( x, 'foo', 5 );
// returns -1
`
*
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]
---
See [LICENSE][stdlib-license].
Copyright © 2016-2026. The Stdlib [Authors][stdlib-authors].
[npm-image]: http://img.shields.io/npm/v/@stdlib/array-base-index-of.svg
[npm-url]: https://npmjs.org/package/@stdlib/array-base-index-of
[test-image]: https://github.com/stdlib-js/array-base-index-of/actions/workflows/test.yml/badge.svg?branch=v0.3.1
[test-url]: https://github.com/stdlib-js/array-base-index-of/actions/workflows/test.yml?query=branch:v0.3.1
[coverage-image]: https://img.shields.io/codecov/c/github/stdlib-js/array-base-index-of/main.svg
[coverage-url]: https://codecov.io/github/stdlib-js/array-base-index-of?branch=main
[chat-image]: https://img.shields.io/badge/zulip-join_chat-brightgreen.svg
[chat-url]: https://stdlib.zulipchat.com
[stdlib]: https://github.com/stdlib-js/stdlib
[stdlib-authors]: https://github.com/stdlib-js/stdlib/graphs/contributors
[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/array-base-index-of/tree/deno
[deno-readme]: https://github.com/stdlib-js/array-base-index-of/blob/deno/README.md
[umd-url]: https://github.com/stdlib-js/array-base-index-of/tree/umd
[umd-readme]: https://github.com/stdlib-js/array-base-index-of/blob/umd/README.md
[esm-url]: https://github.com/stdlib-js/array-base-index-of/tree/esm
[esm-readme]: https://github.com/stdlib-js/array-base-index-of/blob/esm/README.md
[branches-url]: https://github.com/stdlib-js/array-base-index-of/blob/main/branches.md
[stdlib-license]: https://raw.githubusercontent.com/stdlib-js/array-base-index-of/main/LICENSE