Return the first index at which a given element can be found.
npm install @stdlib/utils-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 first index at which a given element can be found.
``bash`
npm install @stdlib/utils-index-of
`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
`
- 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
`
`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
`
*
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/utils-index-of.svg
[npm-url]: https://npmjs.org/package/@stdlib/utils-index-of
[test-image]: https://github.com/stdlib-js/utils-index-of/actions/workflows/test.yml/badge.svg?branch=v0.2.3
[test-url]: https://github.com/stdlib-js/utils-index-of/actions/workflows/test.yml?query=branch:v0.2.3
[coverage-image]: https://img.shields.io/codecov/c/github/stdlib-js/utils-index-of/main.svg
[coverage-url]: https://codecov.io/github/stdlib-js/utils-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/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
[umd-readme]: https://github.com/stdlib-js/utils-index-of/blob/umd/README.md
[esm-url]: https://github.com/stdlib-js/utils-index-of/tree/esm
[esm-readme]: https://github.com/stdlib-js/utils-index-of/blob/esm/README.md
[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