npm package: @stdlib/utils-async-inmap-right
Description: Invoke a function for each element in a collection and update the collection in-place, iterating from right to left.
Author: The Stdlib Authors
Latest version: 0.2.3

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  <summary>
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# inmapRightAsync

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

> Invoke a function for each element in a collection and update the collection in-place, 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-async-inmap-right
```

</section>

<section class="usage">

## Usage

```javascript
var inmapRightAsync = require( '@stdlib/utils-async-inmap-right' );
```

#### inmapRightAsync( collection, \[options,] fcn, done )

Invokes a function for each element in a `collection` and updates the `collection` in-place, iterating from right to left.

<!-- eslint-disable no-use-before-define -->

```javascript
function fcn( value, index, next ) {
    setTimeout( onTimeout, value );
    function onTimeout() {
        console.log( value );
        next( null, value*index );
    }
}

function done( error, collection ) {
    if ( error ) {
        throw error;
    }
    console.log( collection === arr );
    console.log( collection );
}

var arr = [ 1000, 2500, 3000 ];

inmapRightAsync( arr, fcn, done );
/*
    1000
    2500
    3000
    true
    [ 0, 2500, 6000 ]
*/
```

The `next` callback accepts two arguments: `error` and `result`. The second argument to the `next` callback is used to update the `collection` element for the corresponding collection `index`, thus mutating the input `collection`.

```javascript
function fcn( value, index, next ) {
    setTimeout( onTimeout, value );
    function onTimeout() {
        next( null, 'beep: '+index );
    }
}

function done( error, collection ) {
    if ( error ) {
        throw error;
    }
    console.log( collection );
    // => [ 'beep: 0', 'beep: 1', 'beep: 2' ]
}

var arr = [ 1000, 2500, 3000 ];

inmapRightAsync( arr, fcn, done );
```

If the `next` callback is called with an `error` argument, the input `collection` may be **partially** mutated.

```javascript
function fcn( value, index, next ) {
    setTimeout( onTimeout, value );
    function onTimeout() {
        if ( index === 1 ) {
            return next( new Error( 'boop' ) );
        }
        next( null, 'beep: '+index );
    }
}

function done( error, collection ) {
    if ( error ) {
        // Ignore error...
    }
    console.log( collection );
    // => [ 1000, 2000, 'beep: 2' ]
}

var arr = [ 1000, 2000, 3000 ];

inmapRightAsync( arr, fcn, done );
```

The function accepts the following `options`:

-   **limit**: the maximum number of pending invocations at any one time. Default: `infinity`.
-   **series**: boolean indicating whether to sequentially invoke `fcn` for each `collection` element. If `true`, the function sets `options.limit=1`. Default: `false`.
-   **thisArg**: the execution context for `fcn`.

By default, all elements are processed concurrently, which means that the function does **not** guarantee completion order. To process each `collection` element sequentially, set the `series` option to `true`.

<!-- eslint-disable no-use-before-define -->

```javascript
function fcn( value, index, next ) {
    setTimeout( onTimeout, value );
    function onTimeout() {
        console.log( value );
        next( null, value*index );
    }
}

function done( error, collection ) {
    if ( error ) {
        throw error;
    }
    console.log( collection === arr );
    console.log( collection );
}

var arr = [ 1000, 2500, 3000 ];

var opts = {
    'series': true
};

inmapRightAsync( arr, opts, fcn, done );
/* =>
    3000
    2500
    1000
    true
    [ 0, 2500, 6000 ]
*/
```

To limit the maximum number of pending function invocations, set the `limit` option.

<!-- eslint-disable no-use-before-define -->

```javascript
function fcn( value, index, next ) {
    setTimeout( onTimeout, value );
    function onTimeout() {
        console.log( value );
        next( null, value*index );
    }
}

function done( error, collection ) {
    if ( error ) {
        throw error;
    }
    console.log( collection === arr );
    console.log( collection );
}

var arr = [ 1000, 2500, 3000 ];

var opts = {
    'limit': 2
};

inmapRightAsync( arr, opts, fcn, done );
/* =>
    2500
    3000
    1000
    true,
    [ 0, 2500, 6000 ]
*/
```

To set the execution context of `fcn`, set the `thisArg` option.

<!-- eslint-disable no-use-before-define -->

```javascript
function fcn( value, index, next ) {
    this.count += 1;
    setTimeout( onTimeout, value );
    function onTimeout() {
        next( null, value*index );
    }
}

var arr = [ 1000, 2500, 3000 ];

var context = {
    'count': 0
};

var opts = {
    'thisArg': context
};

inmapRightAsync( arr, opts, fcn, done );

function done( error, collection ) {
    if ( error ) {
        throw error;
    }
    console.log( collection === arr );
    // => true

    console.log( collection );
    // => [ 0, 2500, 6000 ]

    console.log( context.count );
    // => 3
}
```

When invoked, `fcn` is provided a maximum of four arguments:

-   **value**: collection value.
-   **index**: collection index.
-   **collection**: the input `collection`.
-   **next**: a callback which should be called once `fcn` has finished processing a collection `value`.

The actual number of provided arguments depends on function `length`. If `fcn` accepts two arguments, `fcn` is provided `value` and `next`. If `fcn` accepts three arguments, `fcn` is provided `value`, `index`, and `next`. For every other `fcn` signature, `fcn` is provided all four arguments.

<!-- eslint-disable no-use-before-define -->

```javascript
function fcn( value, i, collection, next ) {
    console.log( 'collection: %s. %d: %d', collection.join( ',' ), i, value );
    setTimeout( onTimeout, value );
    function onTimeout() {
        console.log( value );
        next( null, value*i );
    }
}

function done( error, collection ) {
    if ( error ) {
        throw error;
    }
    console.log( collection === arr );
    console.log( collection );
}

var arr = [ 1000, 2500, 3000 ];

inmapRightAsync( arr, fcn, done );
/* =>
    collection: 3000,2500,1000. 2: 3000
    collection: 3000,2500,1000. 1: 2500
    collection: 3000,2500,1000. 0: 1000
    1000
    2500
    3000
    true
    [ 0, 2500, 6000 ]
*/
```

#### inmapRightAsync.factory( \[options,] fcn )

Returns a function which invokes a function once for each element in a `collection`, iterating from right to left.

```javascript
function fcn( value, index, next ) {
    setTimeout( onTimeout, value );
    function onTimeout() {
        console.log( value );
        next( null, value*index );
    }
}

function done( error, collection ) {
    if ( error ) {
        throw error;
    }
    console.log( collection );
}

var f = inmapRightAsync.factory( fcn );

var arr1 = [ 1000, 2500, 3000 ];

f( arr1, done );
/* =>
    1000
    2500
    3000
    [ 0, 2500, 6000 ]
*/

var arr2 = [ 100, 250, 300 ];

f( arr2, done );
/* =>
    100
    250
    300
    [ 0, 250, 600 ]
*/
```

The function accepts the same `options` as `inmapRightAsync()`.

</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`).
-   If a provided function calls the `next` callback with a truthy `error` argument, the function suspends execution and immediately calls the `done` callback for subsequent `error` handling.
-   The function invokes the `done` callback with the input `collection` provided as the second argument.
-   The function modifies `collection` elements in-place.
-   The function does **not** support dynamic `collection` resizing.
-   The function does **not** skip `undefined` elements.
-   **Neither** `inmapRightAsync` nor the function returned by the `factory` method **guarantee** asynchronous execution. To guarantee asynchrony, wrap the `done` callback in a function which either executes at the end of the current stack (e.g., `nextTick`) or during a subsequent turn of the event loop (e.g., `setImmediate`, `setTimeout`).

</section>

<!-- /.notes -->

<!-- Package usage examples. -->

<section class="examples">

## Examples

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

```javascript
var resolve = require( 'path' ).resolve;
var readFile = require( '@stdlib/fs-read-file' );
var inmapRightAsync = require( '@stdlib/utils-async-inmap-right' );

var files = [
    resolve( __dirname, 'package.json' ),
    resolve( __dirname, 'README.md' )
];

function done( error, results ) {
    if ( error ) {
        throw error;
    }
    console.log( results );
}

function read( file, next ) {
    var opts = {
        'encoding': 'utf8'
    };
    readFile( file, opts, onFile );

    function onFile( error, data ) {
        if ( error ) {
            error = new Error( 'unable to read file: '+file );
            return next( error );
        }
        next( null, data );
    }
}

inmapRightAsync( files, read, done );
```

</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-async/for-each-right`][@stdlib/utils/async/for-each-right]</span><span class="delimiter">: </span><span class="description">invoke a function once for each element in a collection, iterating from right to left.</span>
-   <span class="package-name">[`@stdlib/utils-async/inmap`][@stdlib/utils/async/inmap]</span><span class="delimiter">: </span><span class="description">invoke a function for each element in a collection and update the collection in-place.</span>
-   <span class="package-name">[`@stdlib/utils-inmap-right`][@stdlib/utils/inmap-right]</span><span class="delimiter">: </span><span class="description">invoke a function for each element in a collection and update the collection in-place, iterating from right to left.</span>

</section>

<!-- /.related -->

<!-- Section for all links. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->


<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-license]: https://raw.githubusercontent.com/stdlib-js/utils-async-inmap-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

<!-- <related-links> -->

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

[@stdlib/utils/async/inmap]: https://www.npmjs.com/package/@stdlib/utils-async-inmap

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

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