About stdlib...

We believe in a future in which the web is a preferred environment for numerical computation. To help realize this future, we've built stdlib. stdlib is a standard library, with an emphasis on numerical and scientific computation, written in JavaScript (and C) for execution in browsers and in Node.js.

The library is fully decomposable, being architected in such a way that you can swap out and mix and match APIs and functionality to cater to your exact preferences and use cases.

When you use stdlib, you can be absolutely certain that you are using the most thorough, rigorous, well-written, studied, documented, tested, measured, and high-quality code out there.

To join us in bringing numerical computing to the web, get started by checking us out on GitHub, and please consider financially supporting stdlib. We greatly appreciate your continued support!

countByAsync

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

Usage

To use in Observable,

countByAsync = require( 'https://cdn.jsdelivr.net/gh/stdlib-js/utils-async-count-by@umd/browser.js' )

To vendor stdlib functionality and avoid installing dependency trees for Node.js, you can use the UMD server build:

var countByAsync = require( 'path/to/vendor/umd/utils-async-count-by/index.js' )

To include the bundle in a webpage,

<script type="text/javascript" src="https://cdn.jsdelivr.net/gh/stdlib-js/utils-async-count-by@umd/browser.js"></script>

If no recognized module system is present, access bundle contents via the global scope:

<script type="text/javascript">
(function () {
    window.countByAsync;
})();
</script>

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.

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.

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.

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.

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.

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.

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().

Notes

A collection may be either an Array, Typed Array, or an array-like 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,

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.

Examples

<!DOCTYPE html>
<html lang="en">
<body>
<script type="text/javascript">
(function () {
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 );

})();
</script>
</body>
</html>

Notice

This package is part of 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, see the main project repository.

Community

License

See LICENSE.

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

README.md

README.md

countByAsync

Usage

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

countByAsync.factory( [options,] indicator )

Notes

Examples

See Also

Notice

Community

License

Copyright

Files

README.md

Latest commit

History

README.md

File metadata and controls

countByAsync

Usage

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

countByAsync.factory( [options,] indicator )

Notes

Examples

See Also

Notice

Community

License

Copyright