reduceRightAsync

Apply a function against an accumulator and each element in a collection and return the accumulated result, iterating from right to left.

Usage

var reduceRightAsync = require( '@stdlib/utils/async/reduce-right' );

reduceRightAsync( collection, initial, [options,] reducer, done )

Applies a function against an accumulator and each element in a collection and returns the accumulated result, iterating from right to left.

function reducer( acc, value, index, next ) {
    setTimeout( onTimeout, value );
    function onTimeout() {
        console.log( value );
        acc.sum += value;
        next( null, acc );
    }
}

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

var arr = [ 1000, 2500, 3000 ];
var acc = {
    'sum': 0
};
reduceRightAsync( arr, acc, reducer, done );
/*
    3000
    2500
    1000
    6500
*/

The next callback accepts two arguments: error and accumulator. The second argument to the next callback is passed as the first argument to the provided reducer.

function reducer( acc, value, index, next ) {
    setTimeout( onTimeout, value );
    function onTimeout() {
        next( null, acc );
    }
}

function done( error, out ) {
    if ( error ) {
        throw error;
    }
    console.log( out === acc );
    // => true
}

var arr = [ 1000, 2500, 3000 ];
var acc = {};

reduceRightAsync( arr, acc, reducer, done );

The function accepts the following options:

limit: the maximum number of pending invocations at any one time. If provided, the function sets options.series=false. Default: infinity.
series: boolean indicating whether to sequentially invoke reducer for each collection element. If true, the function sets options.limit=1. Default: true.
thisArg: the execution context for reducer.

By default, all elements are processed sequentially, which means that the function does guarantee completion order. To process each collection element concurrently, set the series option to false.

function reducer( acc, value, index, next ) {
    setTimeout( onTimeout, value );
    function onTimeout() {
        console.log( value );
        acc.sum += value;
        next( null, acc );
    }
}

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

var arr = [ 1000, 2500, 3000 ];

var acc = {
    'sum': 0
};

var opts = {
    'series': false
};

reduceRightAsync( arr, acc, opts, reducer, done );
/* =>
    1000
    2500
    3000
    6500
*/

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

function reducer( acc, value, index, next ) {
    setTimeout( onTimeout, value );
    function onTimeout() {
        console.log( value );
        acc.sum += value;
        next( null, acc );
    }
}

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

var arr = [ 1000, 2500, 3000 ];

var acc = {
    'sum': 0
};

var opts = {
    'limit': 2
};

reduceRightAsync( arr, acc, opts, reducer, done );
/* =>
    2500
    3000
    1000
    6500
*/

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

function reducer( acc, value, index, next ) {
    this.count += 1;
    setTimeout( onTimeout, value );
    function onTimeout() {
        acc.sum += value;
        next( null, acc );
    }
}

var arr = [ 1000, 2500, 3000 ];

var acc = {
    'sum': 0
};

var context = {
    'count': 0
};

var opts = {
    'thisArg': context
};

reduceRightAsync( arr, acc, opts, reducer, done );

function done( error, acc ) {
    if ( error ) {
        throw error;
    }
    console.log( acc.sum );
    // => 6500

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

When invoked, reducer is provided a maximum of five arguments:

accumulator: accumulated value.
value: collection value.
index: collection index.
collection: the input collection.
next: a callback which should be called once reducer has finished processing a collection value.

The actual number of provided arguments depends on function length. If reducer accepts three arguments, reducer is provided accumulator, value and next. If reducer accepts four arguments, reducer is provided accumulator, value, index, and next. For every other reducer signature, reducer is provided all five arguments.

function reducer( acc, value, i, collection, next ) {
    console.log( 'collection: %s. %d: %d', collection.join( ',' ), i, value );
    setTimeout( onTimeout, value );
    function onTimeout() {
        console.log( value );
        acc.sum += value;
        next( null, acc );
    }
}

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

var arr = [ 1000, 2500, 3000 ];

var acc = {
    'sum': 0
};

reduceRightAsync( arr, acc, reducer, done );
/* =>
    collection: 3000,2500,1000. 2: 3000
    collection: 3000,2500,1000. 1: 2500
    collection: 3000,2500,1000. 0: 1000
    3000
    2500
    1000
    6500
*/

reduceRightAsync.factory( [options,] reducer )

Returns a function which invokes a function once for each element in a collection, iterating from right to left.

function reducer( acc, value, index, next ) {
    setTimeout( onTimeout, value );
    function onTimeout() {
        console.log( value );
        acc.sum += value;
        next( null, acc );
    }
}

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

var f = reduceRightAsync.factory( reducer );

var arr1 = [ 1000, 2500, 3000 ];

var acc1 = {
    'sum': 0
};

f( arr1, acc1, done );
/* =>
    3000
    2500
    1000
    6500
*/

var arr2 = [ 100, 250, 300 ];

var acc2 = {
    'sum': 0
};

f( arr2, acc2, done );
/* =>
    300
    250
    100
    650
*/

The function accepts the same options as reduceRightAsync().

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 invokes the done callback with the accumulator provided as the second argument. If provided an empty collection, the function invokes the done callback with the initial value as the second argument.
The function does not support dynamic collection resizing.
The function does not skip undefined elements.
When processing collection elements concurrently, beware of race conditions when updating an accumulator. This is especially true when an accumulator is a primitive (e.g., a number). In general, prefer object accumulators, as objects are passed by reference, not by value.
Neither reduceRightAsync 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 reduceRightAsync = require( '@stdlib/utils/async/reduce-right' );

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

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

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

    function onFile( error ) {
        if ( error ) {
            return next( null, acc );
        }
        acc.count += 1;
        next( null, acc );
    }
}

var acc = {
    'count': 0
};
reduceRightAsync( files, acc, read, done );