npm package: @stdlib/utils-async-map-function
Description: Invoke a function n times and return an array of accumulated function return values.
Author: The Stdlib Authors
Latest version: 0.2.3

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# mapFunAsync

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> Invoke a function `n` times and return an array of accumulated function return values.

<!-- 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-map-function
```

</section>

<section class="usage">

## Usage

```javascript
var mapFunAsync = require( '@stdlib/utils-async-map-function' );
```

#### mapFunAsync( fcn, n, \[options,] done )

Invokes a function `n` times and returns an `array` of accumulated function return values.

```javascript
function fcn( i, next ) {
    var t = 300 - (i*50);
    setTimeout( onTimeout, t );
    function onTimeout() {
        console.log( i );
        next( null, i );
    }
}

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

mapFunAsync( fcn, 5, done );
/* =>
    4
    3
    2
    1
    0
    [ 0, 1, 2, 3, 4 ]
*/
```

For each iteration, the provided function is invoked with two arguments:

-   `index`: invocation index (starting from zero).
-   `next`: callback to be invoked upon function completion.

The `next` callback accepts two arguments:

-   `error`: error argument.
-   `result`: function result.

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`. If `true`, the function sets `options.limit=1`. Default: `false`.
-   `thisArg`: the execution context for `fcn`.

By default, all invocations are performed concurrently, which means that the function does **not** guarantee completion order. To invoke a function such that only one invocation is pending at any one time, set the `series` option to `true`.

```javascript
function fcn( i, next ) {
    var t = 300 - (i*50);
    setTimeout( onTimeout, t );
    function onTimeout() {
        console.log( i );
        next( null, i );
    }
}

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

var opts = {
    'series': true
};

mapFunAsync( fcn, 5, opts, done );
/* =>
    0
    1
    2
    3
    4
    [ 0, 1, 2, 3, 4 ]
*/
```

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

```javascript
var delays = [ 300, 250, 225, 150, 100 ];

function fcn( i, next ) {
    setTimeout( onTimeout, delays[ i ] );
    function onTimeout() {
        console.log( i );
        next( null, i );
    }
}

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

var opts = {
    'limit': 2
};

mapFunAsync( fcn, 5, opts, done );
/* =>
    1
    0
    3
    2
    4
    [ 0, 1, 2, 3, 4 ]
*/
```

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

<!-- eslint-disable no-invalid-this -->

```javascript
function fcn( i, next ) {
    this.count += 1;
    setTimeout( onTimeout, 0 );
    function onTimeout() {
        next( null, i );
    }
}

function done( error, arr ) {
    if ( error ) {
        throw error;
    }
    console.log( arr );
    // => [ 0, 1, 2, 3, 4 ]
}

var ctx = {
    'count': 0
};
var opts = {
    'thisArg': ctx
};

mapFunAsync( fcn, 5, opts, done );
```

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

Returns a `function` which invokes a function `n` times and returns an `array` of accumulated function return values.

```javascript
function fcn( i, next ) {
    var t = 300 - (i*50);
    setTimeout( onTimeout, t );
    function onTimeout() {
        console.log( i );
        next( null, i );
    }
}

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

var opts = {
    'series': true
};

var f = mapFunAsync.factory( opts, fcn );

f( 5, done );
/* =>
    0
    1
    2
    3
    4
    [ 0, 1, 2, 3, 4 ]
*/

f( 2, done );
/* =>
    0
    1
    [ 0, 1 ]
*/
```

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

</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

-   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.
-   **Neither** `mapFunAsync` 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 randu = require( '@stdlib/random-base-randu' );
var mapFunAsync = require( '@stdlib/utils-async-map-function' );

function rand( i, next ) {
    var t = randu() * i;
    setTimeout( onTimeout, t );
    function onTimeout() {
        next( null, t );
    }
}

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

mapFunAsync( rand, 10, 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>

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<!-- 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-map-function`][@stdlib/utils/map-function]</span><span class="delimiter">: </span><span class="description">invoke a function n times and return an array of accumulated function return values.</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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