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.
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.
reduceRight(callbackFn) reduceRight(callbackFn, initialValue)
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
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 reduceRight()
on an empty array without an initial value creates a TypeError
.
The value that results from the reduction.
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. Read the iterative methods section for more information about how these methods work in general.
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.
The reduceRight()
method is generic. It only expects the this
value to have a length
property and integer-keyed properties.
All caveats about reduce
discussed in when not to use reduce() apply to reduceRight
as well. Because JavaScript has no lazy evaluation semantics, there is no performance difference between reduce
and reduceRight
.
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
).
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
.
const sum = [0, 1, 2, 3].reduceRight((a, b) => a + b); // sum is 6
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); };
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"
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
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
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
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https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reduceRight