Array.prototype.reduceRight()

The reduceRight() method of Array instances 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

reduceRight(callbackFn)
reduceRight(callbackFn, initialValue)

Parameters

callbackFn
- : A function to execute for each element in the array. Its return value becomes the value of the accumulator parameter on the next invocation of callbackFn. For the last invocation, the return value becomes the return value of reduceRight(). The function is called with the following arguments:
  - accumulator
    - : The value resulting from the previous call to callbackFn. On the first call, its value is initialValue if the latter is specified; otherwise its value is the last element of the array.
  - currentValue
    - : The value of the current element. On the first call, its value is the last element if initialValue is specified; otherwise its value is the second-to-last element.
  - currentIndex
    - : The index position of currentValue in the array. On the first call, its value is array.length - 1 if initialValue is specified, otherwise array.length - 2.
  - array
    - : The array reduceRight() was called upon.
initialValue
- : 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 reduceRight() on an empty array without an initial value creates a TypeError.

Return value

The value that results from the reduction.

Description

The reduceRight() method is an iterative method. It runs a "reducer" callback function over all elements in the array, in descending-index order, and accumulates them into a single value.

callbackFn is invoked only for array indexes which have assigned values. It is not invoked for empty slots in sparse arrays.

Unlike other iterative methods, reduceRight() does not accept a thisArg argument. callbackFn is always called with undefined as this, which gets substituted with globalThis if callbackFn is non-strict.

reduceRight() does not mutate the array on which it is called, but the function provided as callbackFn can. Note, however, that the length of the array is saved before the first invocation of callbackFn. Therefore:

callbackFn will not visit any elements added beyond the array's initial length when the call to reduceRight() began.
Changes to already-visited indexes do not cause callbackFn to be invoked on them again.
If an existing, yet-unvisited element of the array is changed by callbackFn, its value passed to the callbackFn will be the value at the time that element gets visited. Deleted elements are not visited.

Warning: Concurrent modifications of the kind described above frequently lead to hard-to-understand code and are generally to be avoided (except in special cases).

The reduceRight() method is generic. It only expects the this value to have a length property and integer-keyed properties.

Examples

How reduceRight() works without an initial value

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

arr.reduceRight((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(
  (accumulator, currentValue, index, array) => accumulator + currentValue,
);

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

	`accumulator`	`currentValue`	`index`	Return value
First call	`4`	`3`	`3`	`7`
Second call	`7`	`2`	`2`	`9`
Third call	`9`	`1`	`1`	`10`
Fourth call	`10`	`0`	`0`	`10`

The array parameter never changes through the process — it's always [0, 1, 2, 3, 4]. The value returned by reduceRight would be that of the last callback invocation (10).

How reduceRight() works with an initial value

Here we reduce the same array using the same algorithm, but with an initialValue of 10 passed as the second argument to reduceRight():

[0, 1, 2, 3, 4].reduceRight(
  (accumulator, currentValue, index, array) => accumulator + currentValue,
  10,
);

	`accumulator`	`currentValue`	`index`	Return value
First call	`10`	`4`	`4`	`14`
Second call	`14`	`3`	`3`	`17`
Third call	`17`	`2`	`2`	`19`
Fourth call	`19`	`1`	`1`	`20`
Fifth call	`20`	`0`	`0`	`20`

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

Sum up all values within an array

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

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); // Logs 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((prev, cur) => prev + cur);
const right = a.reduceRight((prev, cur) => 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

Using reduceRight() with sparse arrays

reduceRight() skips missing elements in sparse arrays, but it does not skip undefined values.

console.log([1, 2, , 4].reduceRight((a, b) => a + b)); // 7
console.log([1, 2, undefined, 4].reduceRight((a, b) => a + b)); // NaN

Calling reduceRight() on non-array objects

The reduceRight() method reads the length property of this and then accesses each property whose key is a nonnegative integer less than length.

const arrayLike = {
  length: 3,
  0: 2,
  1: 3,
  2: 4,
  3: 99, // ignored by reduceRight() since length is 3
};
console.log(Array.prototype.reduceRight.call(arrayLike, (x, y) => x - y));
// -1, which is 4 - 3 - 2