forEachAsync

Invoke a function once for each element in a collection.

Usage

var forEachAsync = require( '@stdlib/utils/async/for-each' );

forEachAsync( collection, [options,] fcn, done )

Invokes a function once for each element in a collection.

function onDuration( value, next ) {
    setTimeout( onTimeout, value );
    function onTimeout() {
        console.log( value );
        next();
    }
}

function done( error ) {
    if ( error ) {
        throw error;
    }
}

var arr = [ 3000, 2500, 1000 ];

forEachAsync( arr, onDuration, done );
/* =>
    1000
    2500
    3000
*/

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.

function onDuration( value, next ) {
    setTimeout( onTimeout, value );
    function onTimeout() {
        console.log( value );
        next();
    }
}

function done( error ) {
    if ( error ) {
        throw error;
    }
}

var arr = [ 3000, 2500, 1000 ];

var opts = {
    'series': true
};

forEachAsync( arr, opts, onDuration, done );
/* =>
    3000
    2500
    1000
*/

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

function onDuration( value, next ) {
    setTimeout( onTimeout, value );
    function onTimeout() {
        console.log( value );
        next();
    }
}

function done( error ) {
    if ( error ) {
        throw error;
    }
}

var arr = [ 3000, 2500, 1000 ];

var opts = {
    'limit': 2
};

forEachAsync( arr, opts, onDuration, done );
/* =>
    2500
    3000
    1000
*/

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

function onDuration( value, next ) {
    this.count += 1;
    setTimeout( onTimeout, value );
    function onTimeout() {
        next();
    }
}

var arr = [ 3000, 2500, 1000 ];

var context = {
    'count': 0
};

var opts = {
    'thisArg': context
};

forEachAsync( arr, opts, onDuration, done );

function done( error ) {
    if ( error ) {
        throw error;
    }
    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.

function onDuration( value, i, collection, next ) {
    console.log( 'collection: %s. %d: %d', collection.join( ',' ), i, value );
    setTimeout( onTimeout, value );
    function onTimeout() {
        console.log( value );
        next();
    }
}

function done( error ) {
    if ( error ) {
        throw error;
    }
}

var arr = [ 3000, 2500, 1000 ];

forEachAsync( arr, onDuration, done );
/* =>
    collection: 3000,2500,1000. 0: 3000
    collection: 3000,2500,1000. 1: 2500
    collection: 3000,2500,1000. 2: 1000
    1000
    2500
    3000
*/

forEachAsync.factory( [options,] fcn )

Returns a function which invokes a function once for each element in a collection.

function onDuration( value, next ) {
    setTimeout( onTimeout, value );
    function onTimeout() {
        console.log( value );
        next();
    }
}

function done( error ) {
    if ( error ) {
        throw error;
    }
}

var f = forEachAsync.factory( onDuration );

var arr1 = [ 3000, 2500, 1000 ];

f( arr1, done );
/* =>
    1000
    2500
    3000
*/

var arr2 = [ 300, 250, 100 ];

f( arr2, done );
/* =>
    100
    250
    300
*/

The function accepts the same options as forEachAsync().

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.
Neither forEachAsync 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).

Examples

var resolve = require( 'path' ).resolve;
var readFile = require( '@stdlib/fs/read-file' );
var forEachAsync = require( '@stdlib/utils/async/for-each' );

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

function done( error ) {
    if ( error ) {
        throw error;
    }
    console.log( 'Successfully read all files.' );
}

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

    function onFile( error ) {
        if ( error ) {
            error = new Error( 'unable to read file: '+file );
            return next( error );
        }
        console.log( 'Successfully read file: %s', file );
        next();
    }
}

forEachAsync( files, read, done );