418 lines
10 KiB
Markdown
418 lines
10 KiB
Markdown
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<!--
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@license Apache-2.0
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Copyright (c) 2018 The Stdlib Authors.
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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-->
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# reduceRightAsync
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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.
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<!-- Section to include introductory text. Make sure to keep an empty line after the intro `section` element and another before the `/section` close. -->
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<section class="intro">
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</section>
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<!-- /.intro -->
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<!-- Package usage documentation. -->
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<section class="usage">
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## Usage
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```javascript
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var reduceRightAsync = require( '@stdlib/utils/async/reduce-right' );
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```
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#### reduceRightAsync( collection, initial, \[options,] reducer, done )
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Applies a `function` against an accumulator and each element in a `collection` and returns the accumulated result, iterating from right to left.
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```javascript
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function reducer( acc, value, index, next ) {
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setTimeout( onTimeout, value );
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function onTimeout() {
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console.log( value );
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acc.sum += value;
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next( null, acc );
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}
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}
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function done( error, acc ) {
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if ( error ) {
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throw error;
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}
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console.log( acc.sum );
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}
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var arr = [ 1000, 2500, 3000 ];
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var acc = {
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'sum': 0
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};
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reduceRightAsync( arr, acc, reducer, done );
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/*
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3000
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2500
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1000
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6500
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*/
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```
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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`.
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<!-- eslint-disable no-use-before-define -->
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```javascript
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function reducer( acc, value, index, next ) {
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setTimeout( onTimeout, value );
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function onTimeout() {
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next( null, acc );
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}
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}
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function done( error, out ) {
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if ( error ) {
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throw error;
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}
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console.log( out === acc );
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// => true
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}
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var arr = [ 1000, 2500, 3000 ];
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var acc = {};
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reduceRightAsync( arr, acc, reducer, done );
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```
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The function accepts the following `options`:
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- `limit`: the maximum number of pending invocations at any one time. If provided, the function sets `options.series=false`. Default: `infinity`.
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- `series`: `boolean` indicating whether to sequentially invoke `reducer` for each `collection` element. If `true`, the function sets `options.limit=1`. Default: `true`.
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- `thisArg`: the execution context for `reducer`.
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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`.
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```javascript
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function reducer( acc, value, index, next ) {
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setTimeout( onTimeout, value );
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function onTimeout() {
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console.log( value );
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acc.sum += value;
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next( null, acc );
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}
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}
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function done( error, acc ) {
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if ( error ) {
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throw error;
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}
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console.log( acc.sum );
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}
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var arr = [ 1000, 2500, 3000 ];
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var acc = {
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'sum': 0
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};
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var opts = {
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'series': false
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};
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reduceRightAsync( arr, acc, opts, reducer, done );
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/* =>
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1000
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2500
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3000
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6500
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*/
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```
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To limit the maximum number of pending function invocations, set the `limit` option.
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```javascript
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function reducer( acc, value, index, next ) {
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setTimeout( onTimeout, value );
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function onTimeout() {
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console.log( value );
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acc.sum += value;
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next( null, acc );
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}
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}
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function done( error, acc ) {
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if ( error ) {
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throw error;
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}
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console.log( acc.sum );
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}
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var arr = [ 1000, 2500, 3000 ];
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var acc = {
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'sum': 0
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};
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var opts = {
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'limit': 2
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};
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reduceRightAsync( arr, acc, opts, reducer, done );
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/* =>
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2500
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3000
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1000
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6500
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*/
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```
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To set the execution context of `reducer`, set the `thisArg` option.
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```javascript
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function reducer( acc, value, index, next ) {
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this.count += 1;
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setTimeout( onTimeout, value );
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function onTimeout() {
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acc.sum += value;
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next( null, acc );
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}
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}
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var arr = [ 1000, 2500, 3000 ];
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var acc = {
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'sum': 0
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};
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var context = {
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'count': 0
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};
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var opts = {
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'thisArg': context
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};
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reduceRightAsync( arr, acc, opts, reducer, done );
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function done( error, acc ) {
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if ( error ) {
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throw error;
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}
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console.log( acc.sum );
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// => 6500
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console.log( context.count );
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// => 3
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}
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```
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When invoked, `reducer` is provided a maximum of five arguments:
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- `accumulator`: accumulated value.
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- `value`: collection value.
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- `index`: collection index.
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- `collection`: the input `collection`.
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- `next`: a callback which should be called once `reducer` has finished processing a collection `value`.
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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.
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```javascript
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function reducer( acc, value, i, collection, next ) {
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console.log( 'collection: %s. %d: %d', collection.join( ',' ), i, value );
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setTimeout( onTimeout, value );
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function onTimeout() {
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console.log( value );
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acc.sum += value;
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next( null, acc );
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}
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}
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function done( error, acc ) {
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if ( error ) {
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throw error;
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}
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console.log( acc.sum );
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}
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var arr = [ 1000, 2500, 3000 ];
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var acc = {
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'sum': 0
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};
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reduceRightAsync( arr, acc, reducer, done );
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/* =>
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collection: 3000,2500,1000. 2: 3000
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collection: 3000,2500,1000. 1: 2500
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collection: 3000,2500,1000. 0: 1000
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3000
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2500
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1000
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6500
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*/
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```
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#### reduceRightAsync.factory( \[options,] reducer )
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Returns a `function` which invokes a function once for each element in a `collection`, iterating from right to left.
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```javascript
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function reducer( acc, value, index, next ) {
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setTimeout( onTimeout, value );
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function onTimeout() {
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console.log( value );
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acc.sum += value;
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next( null, acc );
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}
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}
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function done( error, acc ) {
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if ( error ) {
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throw error;
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}
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console.log( acc.sum );
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}
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var f = reduceRightAsync.factory( reducer );
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var arr1 = [ 1000, 2500, 3000 ];
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var acc1 = {
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'sum': 0
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};
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f( arr1, acc1, done );
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/* =>
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6500
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*/
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var arr2 = [ 100, 250, 300 ];
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var acc2 = {
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'sum': 0
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};
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f( arr2, acc2, done );
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/* =>
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300
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250
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100
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650
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*/
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```
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The function accepts the same `options` as `reduceRightAsync()`.
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</section>
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<!-- /.usage -->
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<!-- Package usage notes. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->
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<section class="notes">
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## Notes
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- 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`).
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- 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.
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- 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.
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- The function does **not** support dynamic `collection` resizing.
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- The function does **not** skip `undefined` elements.
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- 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.
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- **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`).
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</section>
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<!-- /.notes -->
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<!-- Package usage examples. -->
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<section class="examples">
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## Examples
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<!-- eslint no-undef: "error" -->
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```javascript
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var resolve = require( 'path' ).resolve;
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var readFile = require( '@stdlib/fs/read-file' );
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var reduceRightAsync = require( '@stdlib/utils/async/reduce-right' );
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var files = [
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resolve( __dirname, 'package.json' ),
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resolve( __dirname, 'README.md' )
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];
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function done( error, acc ) {
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if ( error ) {
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throw error;
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}
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console.log( acc.count );
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}
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function read( acc, file, next ) {
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var opts = {
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'encoding': 'utf8'
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};
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readFile( file, opts, onFile );
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function onFile( error ) {
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if ( error ) {
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return next( null, acc );
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}
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acc.count += 1;
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next( null, acc );
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}
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}
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var acc = {
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'count': 0
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};
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reduceRightAsync( files, acc, read, done );
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```
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</section>
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<!-- /.examples -->
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<!-- 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. -->
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<section class="references">
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</section>
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<!-- /.references -->
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<!-- Section for all links. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->
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<section class="links">
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[mdn-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array
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[mdn-typed-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray
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[mdn-object]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object
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</section>
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<!-- /.links -->
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