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 source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.
See also Array.prototype.reduce()
for left-to-right.
Syntax
arr.reduceRight(callback(accumulator, currentValue[, index[, array]])[, initialValue])
Parameters
callback
- Function to execute on each value in the array, taking four 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
Optional- The index of the current element being processed in the array.
array
Optional- The array
reduceRight()
was called upon.
initialValue
Optional- Value to use as accumulator to the first call of the
callback
. 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 callback
would look something like this:
array.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 callback
.
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
.
Polyfill
reduceRight
was added to the ECMA-262 standard in the 5th edition; as such it may not be present in all implementations of the standard. You can work around this by inserting the following code at the beginning of your scripts, allowing use of reduceRight
in implementations which do not natively support it.
// Production steps of ECMA-262, Edition 5, 15.4.4.22 // Reference: http://es5.github.io/#x15.4.4.22 if ('function' !== typeof Array.prototype.reduceRight) { Array.prototype.reduceRight = function(callback /*, initialValue*/) { 'use strict'; if (null === this || 'undefined' === typeof this) { throw new TypeError('Array.prototype.reduce called on null or undefined'); } if ('function' !== typeof callback) { throw new TypeError(callback + ' is not a function'); } var t = Object(this), len = t.length >>> 0, k = len - 1, value; if (arguments.length >= 2) { value = arguments[1]; } else { while (k >= 0 && !(k in t)) { k--; } if (k < 0) { throw new TypeError('Reduce of empty array with no initial value'); } value = t[k--]; } for (; k >= 0; k--) { if (k in t) { value = callback(value, t[k], k, t); } } return value; }; }
Examples
Sum up all values within an array
var sum = [0, 1, 2, 3].reduceRight(function(a, b) { return a + b; }); // sum is 6
Flatten an array of arrays
var 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); }
โโโโโโDifference between reduce
and reduceRight
var a = ['1', '2', '3', '4', '5']; var left = a.reduce(function(prev, cur) { return prev + cur; }); var right = a.reduceRight(function(prev, cur) { return prev + cur; }); console.log(left); // "12345" console.log(right); // "54321"
Defining Composible Function
The concept of compose function is simple it combines n functions. Itโs a flowing right-to-left, calling each function with the output of the last one.
/** * Function Composition is way in which result of one function can * be passed to another and so on. * * h(x) = f(g(x)) * * Function execution happens right to left * * https://en.wikipedia.org/wiki/Function_composition */ 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 (ECMA-262) The definition of 'Array.prototype.reduceRight' in that specification. |
Browser compatibility
Desktop | Mobile | Server | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
reduceRight | Chrome Full support 3 | Edge Full support 12 | Firefox Full support 3 | IE Full support 9 | Opera Full support 10.5 | Safari Full support 5 | WebView Android Full support โค37 | Chrome Android Full support 18 | Firefox Android Full support 4 | Opera Android Full support 14 | Safari iOS Full support 4 | Samsung Internet Android Full support 1.0 | nodejs Full support 0.1.100 |
Legend
- Full support
- Full support