npm package: @stdlib/utils-while-each-right
Description: While a test condition is true, invoke a function for each element in a collection, iterating from right to left.
Author: The Stdlib Authors
Latest version: 0.2.3

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# whileEachRight

[![NPM version][npm-image]][npm-url] [![Build Status][test-image]][test-url] [![Coverage Status][coverage-image]][coverage-url] <!-- [![dependencies][dependencies-image]][dependencies-url] -->

> While a test condition is true, invoke a function for each element in a collection, iterating from right to left.

<!-- Section to include introductory text. Make sure to keep an empty line after the intro `section` element and another before the `/section` close. -->

<section class="intro">

</section>

<!-- /.intro -->

<!-- Package usage documentation. -->

<section class="installation">

## Installation

```bash
npm install @stdlib/utils-while-each-right
```

</section>

<section class="usage">

## Usage

```javascript
var whileEachRight = require( '@stdlib/utils-while-each-right' );
```

#### whileEachRight( collection, predicate, fcn\[, thisArg ] )

Invokes a `function` for each element in a `collection`, iterating from right to left, until either a `predicate` function returns `false` or the function has iterated over all `collection` elements.

```javascript
function predicate( value ) {
    return ( value === value );
}

function log( value, index ) {
    console.log( '%s: %d', index, value );
}

var arr = [ 1, NaN, 2, 3, 4 ];

whileEachRight( arr, predicate, log );
/* =>
    4: 4
    3: 3
    2: 2
*/
```

Both the `predicate` function and the `function` to apply are provided three arguments:

-   `value`: collection element.
-   `index`: collection index.
-   `collection`: input collection.

Basic support for dynamic collections is provided. Note, however, that index incrementation is **not** guaranteed to be monotonically **decreasing**.

```javascript
var arr = [ 1, 2, 3, 4 ];
var i = 0;

function predicate( value ) {
    return ( value === value );
}

function log1( value, index, collection ) {
    console.log( '%s: %d', index, value );
    i += 1;
    if ( index === 0 && collection.length < 10 ) {
        collection.unshift( i+1 );
    }
}

whileEachRight( arr, predicate, log1 );
/* =>
    3: 4
    2: 3
    1: 2
    0: 1
    0: 5
    0: 6
    0: 7
    0: 8
    0: 9
    0: 10
*/

function log2( value, index, collection ) {
    console.log( '%s: %d', index, value );
    collection.pop();
}

arr = [ 1, 2, 3, 4 ];

whileEachRight( arr, predicate, log2 );
/* =>
    3: 4
    1: 2
*/
```

To set the function execution context for the applied function, provide a `thisArg`.

```javascript
function predicate( value ) {
    return ( value === value );
}

function sum( value ) {
    this.sum += value;
    this.count += 1;
}

var arr = [ 1, NaN, 2, 3, 4 ];

var context = {
    'sum': 0,
    'count': 0
};

whileEachRight( arr, predicate, sum, context );

var mean = context.sum / context.count;
// returns 3.0
```

</section>

<!-- /.usage -->

<!-- Package usage notes. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->

<section class="notes">

## Notes

-   A `collection` may be either an [`Array`][mdn-array], [`Typed Array`][mdn-typed-array], or an array-like [`Object`][mdn-object] (excluding `strings` and `functions`).

-   The function returns the input `collection`.

-   The function does **not** skip `undefined` elements.

    <!-- eslint-disable no-sparse-arrays -->

    ```javascript
    function predicate( value ) {
        return ( value === value );
    }

    function log( value, index ) {
        console.log( '%s: %s', index, value );
    }

    var arr = [ 1, , , 4 ];

    whileEachRight( arr, predicate, log );
    /* =>
        3: 4
        2: undefined
        1: undefined
        0: 1
    */
    ```

-   The function provides limited support for dynamic collections (i.e., collections whose `length` changes during execution).

</section>

<!-- /.notes -->

<!-- Package usage examples. -->

<section class="examples">

## Examples

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

```javascript
var isEven = require( '@stdlib/assert-is-even' ).isPrimitive;
var randu = require( '@stdlib/random-base-randu' );
var floor = require( '@stdlib/math-base-special-floor' );
var whileEachRight = require( '@stdlib/utils-while-each-right' );

var arr;
var i;
var j;

function predicate( value ) {
    return ( value === value );
}

function log( value, index, collection ) {
    console.log( '%s: %d', index, value );
    i += 1;
    if ( isEven( i ) ) {
        collection.pop();
    } else {
        collection.unshift( i+1 );
    }
}

arr = new Array( 100 );
j = floor( randu()*arr.length );
for ( i = arr.length-1; i >= 0; i-- ) {
    if ( i === j ) {
        arr[ i ] = NaN;
    } else {
        arr[ i ] = i;
    }
}

i = 0;
whileEachRight( arr, predicate, log );
```

</section>

<!-- /.examples -->

<!-- Section to include cited references. If references are included, add a horizontal rule *before* the section. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->

<section class="references">

</section>

<!-- /.references -->

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

<section class="related">

* * *

## See Also

-   <span class="package-name">[`@stdlib/utils-while-each`][@stdlib/utils/while-each]</span><span class="delimiter">: </span><span class="description">while a test condition is true, invoke a function for each element in a collection.</span>
-   <span class="package-name">[`@stdlib/utils-until-each-right`][@stdlib/utils/until-each-right]</span><span class="delimiter">: </span><span class="description">until a test condition is true, invoke a function for each element in a collection, iterating from right to left.</span>

</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>

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

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[stdlib-license]: https://raw.githubusercontent.com/stdlib-js/utils-while-each-right/main/LICENSE

[mdn-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array

[mdn-typed-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray

[mdn-object]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object

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[@stdlib/utils/while-each]: https://www.npmjs.com/package/@stdlib/utils-while-each

[@stdlib/utils/until-each-right]: https://www.npmjs.com/package/@stdlib/utils-until-each-right

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