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.
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.
// 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)
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
.
The value that results from the reduction.
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((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
).
And if you were to provide an initialValue
, the result would look like this:
[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 arrays = [ [0, 1], [2, 3], [4, 5], ]; const flattened = arrays.reduceRight((a, b) => a.concat(b), []); // flattened is [4, 5, 2, 3, 0, 1]
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); };
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
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reduceRight |
3 |
12 |
3 |
9 |
10.5 |
5 |
≤37 |
18 |
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14 |
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0.10.0 |
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https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reduceRight