npm package: @stdlib/utils-async-compose
Description: Function composition.
Author: The Stdlib Authors
Latest version: 0.2.3

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  <summary>
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</details>

# composeAsync

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

> [Function composition][function-composition].

<!-- 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>

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<!-- Package usage documentation. -->

<section class="installation">

## Installation

```bash
npm install @stdlib/utils-async-compose
```

</section>

<section class="usage">

## Usage

```javascript
var composeAsync = require( '@stdlib/utils-async-compose' );
```

#### composeAsync( ...fcn )

Returns a composite function. Starting from the right, the composite function evaluates each function and passes the result as the first argument to the next function. The result of the leftmost function is the result of the whole.

```javascript
function a( x, next ) {
    setTimeout( onTimeout, 0 );
    function onTimeout() {
        next( null, 2*x );
    }
}

function b( x, next ) {
    setTimeout( onTimeout, 0 );
    function onTimeout() {
        next( null, x+3 );
    }
}

function c( x, next ) {
    setTimeout( onTimeout, 0 );
    function onTimeout() {
        next( null, x/5 );
    }
}

var f = composeAsync( c, b, a );

function done( error, result ) {
    if ( error ) {
        throw error;
    }
    console.log( result );
    // => 3
}

f( 6, done ); // ((2*x)+3)/5
```

The last argument provided to each composed function is a `next` callback which accepts two arguments:

-   **error**: error argument.
-   **result**: function result.

**Only** the rightmost function is explicitly permitted to accept multiple arguments. All other functions are evaluated as **binary** functions.

```javascript
function a( x, y, next ) {
    setTimeout( onTimeout, 0 );
    function onTimeout() {
        next( null, (x*5) + (y*3) );
    }
}

function b( r, next ) {
    setTimeout( onTimeout, 0 );
    function onTimeout() {
        next( null, r+12 );
    }
}

var f = composeAsync( b, a );

function done( error, result ) {
    if ( error ) {
        throw error;
    }
    console.log( result );
    // => 50
}

f( 4, 6, done );
```

</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

-   The function will throw if provided fewer than `2` input arguments.
-   If a composed function calls the `next` callback with a truthy `error` argument, the function suspends execution and immediately calls the `done` callback for subsequent `error` handling.
-   Execution is **not** guaranteed to be asynchronous. 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 composeAsync = require( '@stdlib/utils-async-compose' );

function a( x, y, next ) {
    setTimeout( onTimeout, 0 );
    function onTimeout() {
        next( null, x*y );
    }
}

function b( z, next ) {
    setTimeout( onTimeout, 0 );
    function onTimeout() {
        next( null, z+5 );
    }
}

function c( r, next ) {
    setTimeout( onTimeout, 0 );
    function onTimeout() {
        next( null, r/10 );
    }
}

var f = composeAsync( c, b, a );

function done( error, result ) {
    if ( error ) {
        throw error;
    }
    console.log( result );
    // => 2
}

f( 5, 3, done );
```

</section>

<!-- /.examples -->

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</section>

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<section class="related">

* * *

## See Also

-   <span class="package-name">[`@stdlib/utils-compose`][@stdlib/utils/compose]</span><span class="delimiter">: </span><span class="description">function composition.</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].

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

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[function-composition]: https://en.wikipedia.org/wiki/Function_composition_%28computer_science%29

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