Array.prototype.reduceRight()

The reduceRight() method applies a function against an accumulator and each value of the array (from right-to-left) to reduce it to a single value.

See also Array.prototype.reduce() for left-to-right.

Syntax

// Arrow function
reduceRight((accumulator, currentValue) => { ... } )
reduceRight((accumulator, currentValue, index) => { ... } )
reduceRight((accumulator, currentValue, index, array) => { ... } )
reduceRight((accumulator, currentValue, index, array) => { ... }, initialValue)

// Callback function
reduceRight(callbackFn)
reduceRight(callbackFn, initialValue)

// Callback reducer function
reduceRight(function callbackFn(accumulator, currentValue) { ... })
reduceRight(function callbackFn(accumulator, currentValue, index) { ... })
reduceRight(function callbackFn(accumulator, currentValue, index, array){ ... })
reduceRight(function callbackFn(accumulator, currentValue, index, array) { ... }, initialValue)

Parameters

callbackFn

Function to execute on each value in the array, taking four arguments:

accumulator: The value previously returned in the last invocation of the callback, or initialValue, if supplied. (See below.)
currentValue: The current element being processed in the array.
indexOptional: The index of the current element being processed in the array.
arrayOptional: The array reduceRight() was called upon.

initialValue Optional

Value to use as accumulator to the first call of the callbackFn. If no initial value is supplied, the last element in the array will be used and skipped. Calling reduce or reduceRight on an empty array without an initial value creates a TypeError.

Return value

The value that results from the reduction.

Description

reduceRight executes the callback function once for each element present in the array, excluding holes in the array, receiving four arguments: the initial value (or value from the previous callback call), the value of the current element, the current index, and the array over which iteration is occurring.

The call to the reduceRight callbackFn would look something like this:

arr.reduceRight(function(accumulator, currentValue, index, array) {
  // ...
});

The first time the function is called, the accumulator and currentValue can be one of two values. If an initialValue was provided in the call to reduceRight, then accumulator will be equal to initialValue and currentValue will be equal to the last value in the array. If no initialValue was provided, then accumulator will be equal to the last value in the array and currentValue will be equal to the second-to-last value.

If the array is empty and no initialValue was provided, TypeError would be thrown. If the array has only one element (regardless of position) and no initialValue was provided, or if initialValue is provided but the array is empty, the solo value would be returned without calling callbackFn.

Some example run-throughs of the function would look like this:

[0, 1, 2, 3, 4].reduceRight(function(accumulator, currentValue, index, array) {
  return accumulator + currentValue;
});

The callback would be invoked four times, with the arguments and return values in each call being as follows:

`callback`	`accumulator`	`currentValue`	`index`	`array`	return value
first call	`4`	`3`	`3`	`[0, 1, 2, 3, 4]`	`7`
second call	`7`	`2`	`2`	`[0, 1, 2, 3, 4]`	`9`
third call	`9`	`1`	`1`	`[0, 1, 2, 3, 4]`	`10`
fourth call	`10`	`0`	`0`	`[0, 1, 2, 3, 4]`	`10`

The value returned by reduceRight would be that of the last callback invocation (10).

And if you were to provide an initialValue, the result would look like this:

[0, 1, 2, 3, 4].reduceRight(function(accumulator, currentValue, index, array) {
  return accumulator + currentValue;
}, 10);

`callback`	`accumulator`	`currentValue`	`index`	`array`	return value
first call	`10`	`4`	`4`	`[0, 1, 2, 3, 4]`	`14`
second call	`14`	`3`	`3`	`[0, 1, 2, 3, 4]`	`17`
third call	`17`	`2`	`2`	`[0, 1, 2, 3, 4]`	`19`
fourth call	`19`	`1`	`1`	`[0, 1, 2, 3, 4]`	`20`
fifth call	`20`	`0`	`0`	`[0, 1, 2, 3, 4]`	`20`

The value returned by reduceRight this time would be, of course, 20.

Polyfill

reduceRight was added to the ECMA-262 standard in the 5th edition; as such it may not be present in all implementations of the standard. You can work around this by inserting the following code at the beginning of your scripts, allowing use of reduceRight in implementations which do not natively support it.

// Production steps of ECMA-262, Edition 5, 15.4.4.22
// Reference: https://es5.github.io/#x15.4.4.22
if ('function' !== typeof Array.prototype.reduceRight) {
  Array.prototype.reduceRight = function(callback /*, initialValue*/) {
    'use strict';
    if (null === this || 'undefined' === typeof this) {
      throw new TypeError('Array.prototype.reduce called on null or undefined');
    }
    if ('function' !== typeof callback) {
      throw new TypeError(callback + ' is not a function');
    }
    var t = Object(this), len = t.length >>> 0, k = len - 1, value;
    if (arguments.length >= 2) {
      value = arguments[1];
    } else {
      while (k >= 0 && !(k in t)) {
        k--;
      }
      if (k < 0) {
        throw new TypeError('Reduce of empty array with no initial value');
      }
      value = t[k--];
    }
    for (; k >= 0; k--) {
      if (k in t) {
        value = callback(value, t[k], k, t);
      }
    }
    return value;
  };
}

Examples

Sum up all values within an array

var sum = [0, 1, 2, 3].reduceRight(function(a, b) {
  return a + b;
});
// sum is 6

Flatten an array of arrays

var flattened = [[0, 1], [2, 3], [4, 5]].reduceRight(function(a, b) {
    return a.concat(b);
}, []);
// flattened is [4, 5, 2, 3, 0, 1]

Run a list of asynchronous functions with callbacks in series each passing their results to the next

const waterfall = (...functions) => (callback, ...args) =>
  functions.reduceRight(
    (composition, fn) => (...results) => fn(composition, ...results),
    callback
  )(...args);

const randInt = max => Math.floor(Math.random() * max)

const add5 = (callback, x) => {
  setTimeout(callback, randInt(1000), x + 5);
};
const mult3 = (callback, x) => {
  setTimeout(callback, randInt(1000), x * 3);
};
const sub2 = (callback, x) => {
  setTimeout(callback, randInt(1000), x - 2);
};
const split = (callback, x) => {
  setTimeout(callback, randInt(1000), x, x);
};
const add = (callback, x, y) => {
  setTimeout(callback, randInt(1000), x + y);
};
const div4 = (callback, x) => {
  setTimeout(callback, randInt(1000), x / 4);
};

const computation = waterfall(add5, mult3, sub2, split, add, div4);
computation(console.log, 5) // -> 14

// same as:

const computation2 = (input, callback) => {
  const f6 = x=> div4(callback, x);
  const f5 = (x, y) => add(f6, x, y);
  const f4 = x => split(f5, x);
  const f3 = x => sub2(f4, x);
  const f2 = x => mult3(f3, x);
  add5(f2, input);
}

Difference between `reduce` and `reduceRight`

var a = ['1', '2', '3', '4', '5'];
var left  = a.reduce(function(prev, cur)      { return prev + cur; });
var right = a.reduceRight(function(prev, cur) { return prev + cur; });

console.log(left);  // "12345"
console.log(right); // "54321"

Defining composable functions

Function composition is a mechanism for combining functions, in which the output of each function is passed into the next one, and the output of the last function is the final result. In this example we use reduceRight() to implement function composition.

See also Function composition on Wikipedia.

const compose = (...args) => (value) => args.reduceRight((acc, fn) => fn(acc), value)

// Increment passed number
const inc = (n) => n + 1

// Doubles the passed value
const double = (n) => n * 2

// using composition function
console.log(compose(double, inc)(2)); // 6

// using composition function
console.log(compose(inc, double)(2)); // 5

Specifications

Specification
ECMAScript Language Specification (ECMAScript) # sec-array.prototype.reduceright

Browser compatibility

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	WebView Android	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet
`reduceRight`	3	12	3	9	10.5	5	≤37	18	4	14	4	1.0