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Apply a function against an accumulator and each element in a collection and return the accumulated result, iterating from right to left.
npm install @stdlib/utils-async-reduce-rightAlternatively,
- To load the package in a website via a
scripttag without installation and bundlers, use the ES Module available on theesmbranch (see README). - If you are using Deno, visit the
denobranch (see README for usage intructions). - For use in Observable, or in browser/node environments, use the Universal Module Definition (UMD) build available on the
umdbranch (see README).
The branches.md file summarizes the available branches and displays a diagram illustrating their relationships.
To view installation and usage instructions specific to each branch build, be sure to explicitly navigate to the respective README files on each branch, as linked to above.
var reduceRightAsync = require( '@stdlib/utils-async-reduce-right' );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
reducerfor eachcollectionelement. Iftrue, the function setsoptions.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
reducerhas finished processing a collectionvalue.
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
*/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().
- A
collectionmay be either anArray,Typed Array, or an array-likeObject(excludingstringsandfunctions). - If a provided function calls the
nextcallback with a truthyerrorargument, the function suspends execution and immediately calls thedonecallback for subsequenterrorhandling. - The function invokes the
donecallback with theaccumulatorprovided as the second argument. If provided an emptycollection, the function invokes thedonecallback with theinitialvalue as the second argument. - The function does not support dynamic
collectionresizing. - The function does not skip
undefinedelements. - When processing
collectionelements concurrently, beware of race conditions when updating an accumulator. This is especially true when an accumulator is a primitive (e.g., anumber). In general, preferobjectaccumulators, as objects are passed by reference, not by value. - Neither
reduceRightAsyncnor the function returned by thefactorymethod guarantee asynchronous execution. To guarantee asynchrony, wrap thedonecallback 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).
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 );@stdlib/utils-async/for-each-right: invoke a function once for each element in a collection, iterating from right to left.@stdlib/utils-async/reduce: apply a function against an accumulator and each element in a collection and return the accumulated result.@stdlib/utils-reduce-right: apply a function against an accumulator and each element in an array while iterating from right to left and return the accumulated result.
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.
See LICENSE.
Copyright © 2016-2025. The Stdlib Authors.
