array.reduceRight

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

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

// flattened is [4, 5, 2, 3, 0, 1]

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

Syntax

arr.reduceRight(callback[, initialValue])

Parameters

callback

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

previousValue

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.

index

The index of the current element being processed in the array.

array

The array reduce was called upon.

initialValue

Optional. Object to use as the first argument to the first call of the callback.

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(previousValue, currentValue, index, array) {
  // ...
});

The first time the function is called, the previousValue and currentValue can be one of two values. If an initialValue was provided in the call to reduceRight, then previousValue will be equal to initialValue and currentValue will be equal to the last value in the array. If no initialValue was provided, then previousValue 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(previousValue, currentValue, index, array) {
  return previousValue + currentValue;
});

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

callback	previousValue	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(previousValue, currentValue, index, array) {
  return previousValue + currentValue;
}, 10);

callback	previousValue	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.

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]

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"

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;
  };
}

Specifications

Specification	Status	Comment
ECMAScript 5.1 (ECMA-262) The definition of 'Array.prototype.reduceRight' in that specification.	Standard	Initial definition. Implemented in JavaScript 1.8.
ECMAScript 2015 (6th Edition, ECMA-262) The definition of 'Array.prototype.reduceRight' in that specification.	Standard
ECMAScript 2017 Draft (ECMA-262) The definition of 'Array.prototype.reduceRight' in that specification.	Draft

Browser compatibility

See also

• Array.prototype.reduce()