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.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

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

Syntax

arr.reduceRight(callback(accumulator, currentValue[, index[, array]])[, initialValue])

Parameters

callback

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.

initialValueOptional

Value to use as accumulator to the first call of the callback. 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 callback would look something like this:

array.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 callback.

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:

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);

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: http://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 Composible Function

The concept of compose function is simple it combines n functions. It’s a flowing right-to-left, calling each function with the output of the last one.

/**
 * Function Composition is way in which result of one function can
 * be passed to another and so on.
 *
 * h(x) = f(g(x))
 *
 * Function execution happens right to left
 *
 * https://en.wikipedia.org/wiki/Function_composition
 */

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

Browser compatibility

See also

• Array.prototype.reduce()