npm package: @stdlib/utils-async-count-by
Description: Group values according to an indicator function and return group counts.
Author: The Stdlib Authors
Latest version: 0.2.3

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# countByAsync

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> Group values according to an indicator function and return group counts.

<!-- 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-count-by
```

</section>

<section class="usage">

## Usage

```javascript
var countByAsync = require( '@stdlib/utils-async-count-by' );
```

#### countByAsync( collection, \[options,] indicator, done )

Groups values according to an `indicator` function, which specifies which group an element in the input `collection` belongs to, and returns group counts.

```javascript
function indicator( value, next ) {
    setTimeout( onTimeout, value );
    function onTimeout() {
        console.log( value );
        next( null, (value > 2000) );
    }
}

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

var arr = [ 3000, 2500, 1000 ];

countByAsync( arr, indicator, done );
/* =>
    1000
    2500
    3000
    { 'true': 2, 'false': 1 }
*/
```

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

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`.

```javascript
function indicator( value, next ) {
    setTimeout( onTimeout, value );
    function onTimeout() {
        console.log( value );
        next( null, (value > 2000) );
    }
}

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

var arr = [ 3000, 2500, 1000 ];

var opts = {
    'series': true
};

countByAsync( arr, opts, indicator, done );
/* =>
    3000
    2500
    1000
    { 'true': 2, 'false': 1 }
*/
```

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

```javascript
function indicator( value, next ) {
    setTimeout( onTimeout, value );
    function onTimeout() {
        console.log( value );
        next( null, (value > 2000) );
    }
}

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

var arr = [ 3000, 2500, 1000 ];

var opts = {
    'limit': 2
};

countByAsync( arr, opts, indicator, done );
/* =>
    2500
    3000
    1000
    { 'true': 2, 'false': 1 }
*/
```

To set the execution context of the `indicator` function, set the `thisArg` option.

```javascript
function indicator( value, next ) {
    this.count += 1;
    setTimeout( onTimeout, value );
    function onTimeout() {
        next( null, (value > 2000) );
    }
}

var arr = [ 3000, 2500, 1000 ];

var context = {
    'count': 0
};

var opts = {
    'thisArg': context
};

countByAsync( arr, opts, indicator, done );

function done( error, result ) {
    if ( error ) {
        throw error;
    }
    console.log( result );
    // => { 'true': 2, 'false': 1 }

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

When invoked, the `indicator` function 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 the `indicator` function has finished processing a collection `value`.

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

```javascript
function indicator( 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 > 2000) );
    }
}

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

var arr = [ 3000, 2500, 1000 ];

countByAsync( arr, indicator, done );
/* =>
    collection: 3000,2500,1000. 0: 3000
    collection: 3000,2500,1000. 1: 2500
    collection: 3000,2500,1000. 2: 1000
    1000
    2500
    3000
    { 'true': 2, 'false': 1 }
*/
```

#### countByAsync.factory( \[options,] indicator )

Returns a `function` which invokes an `indicator` function once for each element in a `collection` and returns group counts.

```javascript
function indicator( value, next ) {
    setTimeout( onTimeout, value );
    function onTimeout() {
        console.log( value );
        next( null, (value > 2000) );
    }
}

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

var f = countByAsync.factory( indicator );

var arr1 = [ 3000, 2500, 1000 ];

f( arr1, done );
/* =>
    1000
    2500
    3000
    { 'true': 2, 'false': 1 }
*/

var arr2 = [ 300, 250, 100 ];

f( arr2, done );
/* =>
    100
    250
    300
    { 'false': 3 }
*/
```

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

</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 does **not** support dynamic `collection` resizing.

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

-   If provided an empty `collection`, the function calls the `done` callback with an empty `object` for the group results.

-   **Neither** `countByAsync` 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`).

-   The `group` returned by an `indicator` function should be a value which can be serialized as an `object` key. As a counterexample,

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

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

    var arr = [ 3000, 2500, 1000 ];

    countByAsync( arr, indicator, done );
    /* =>
        1000
        2500
        3000
        { '[object Object]': 3 }
    */
    ```

    while each group identifier is unique, all collection elements resolve to the same group because each group identifier serializes to the same `string`.

</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 countByAsync = require( '@stdlib/utils-async-count-by' );

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

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

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

    function onFile( error ) {
        if ( error ) {
            return next( null, 'nonreadable' );
        }
        next( null, 'readable' );
    }
}

countByAsync( files, indicator, 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-count-by`][@stdlib/utils/count-by]</span><span class="delimiter">: </span><span class="description">group values according to an indicator function and return group counts.</span>
-   <span class="package-name">[`@stdlib/utils-async/group-by`][@stdlib/utils/async/group-by]</span><span class="delimiter">: </span><span class="description">group values according to an indicator function.</span>
-   <span class="package-name">[`@stdlib/utils-async/tabulate-by`][@stdlib/utils/async/tabulate-by]</span><span class="delimiter">: </span><span class="description">generate a frequency table according to an indicator function.</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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