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(accumulator, currentValue) { /* ... */ })
reduceRight(function(accumulator, currentValue, index) { /* ... */ })
reduceRight(function(accumulator, currentValue, index, array){ /* ... */ })
reduceRight(function(accumulator, currentValue, index, array) { /* ... */ }, initialValue)

Parameters

callbackFn

Function to execute on each value in the array.

The function is called with the following 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.

index

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

array

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.

Examples

Sum up all values within an array

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

Flatten an array of arrays

const 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

const a = ['1', '2', '3', '4', '5'];
const left  = a.reduce(function(prev, cur)      { return prev + cur; });
const 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
# sec-array.prototype.reduceright

Browser compatibility

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See also