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.
Array.prototype.reduceRight()
Try it
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 aTypeError
.
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
Browser compatibility
Desktop | Mobile | Server | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Chrome | Edge | Firefox | Internet Explorer | Opera | Safari | WebView Android | Chrome Android | Firefox for Android | Opera Android | Safari on IOS | Samsung Internet | Deno | Node.js | |
reduceRight |
3
|
12
|
3
|
9
|
10.5
|
5
|
≤37
|
18
|
4
|
14
|
4
|
1.0
|
1.0
|
0.10.0
|
See also
© 2005–2022 MDN contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reduceRight