lastIndexOf

Return the index of the last element which equals a provided search element.

Installation

npm install @stdlib/array-base-last-index-of

Usage

var lastIndexOf = require( '@stdlib/array-base-last-index-of' );

lastIndexOf( x, searchElement, fromIndex )

Returns the index of the last element which equals a provided search element.

var x = [ 1, 2, 3, 4, 5, 6 ];

var idx = lastIndexOf( x, 3, 5 );
// returns 2

If the function is unable to find an element which equals a provided search element, the function returns -1.

var x = [ 1, 2, 3, 4, 5, 6 ];

var idx = lastIndexOf( x, 7, 5 );
// returns -1

To begin searching from a specific index, provide a corresponding fromIndex argument.

var x = [ 1, 1, 2, 1, 2, 3, 3 ];

var idx = lastIndexOf( x, 2, 3 );
// returns 2

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.

var x = [ 1, 1, 2, 1, 2, 3, 3 ];

var idx = lastIndexOf( x, 2, -4 );
// returns 2

Notes

• The function scans an input array from the starting index to the beginning of the array (i.e., backward).

• If provided an array-like object having a lastIndexOf method, the function defers execution to that method and assumes that the method API has the following signature:

  x.lastIndexOf( searchElement, fromIndex )

• If provided an array-like object without a lastIndexOf 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.

Examples

var lastIndexOf = require( '@stdlib/array-base-last-index-of' );

var x = [ 'foo', 'bar', 'beep', 'boop', 'foo', 'bar' ];

var idx = lastIndexOf( x, 'beep', 5 );
// returns 2

idx = lastIndexOf( x, 'bop', 5 );
// returns -1

idx = lastIndexOf( x, 'foo', 5 );
// returns 4

idx = lastIndexOf( x, 'foo', -3 );
// returns 0

idx = lastIndexOf( x, 'foo', -50 );
// returns -1

Notice

This package is part of 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, see the main project repository.

Community

License

See LICENSE.

Copyright

Copyright © 2016-2026. The Stdlib Authors.