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.
var flattened = [[0, 1], [2, 3], [4, 5]].reduceRight(function(a, b) { return a.concat(b); }, []); // flattened is [4, 5, 2, 3, 0, 1]
See also Array.prototype.reduce()
for left-to-right.
Syntax
arr.reduceRight(callback[, initialValue])
Parameters
callback
- Function to execute on each value in the array, taking four arguments:
previousValue
- 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
reduce
was called upon.
initialValue
- Optional. Object to use as the first argument to the first call of the
callback
.
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(previousValue, currentValue, index, array) { // ... });
The first time the function is called, the previousValue
and currentValue
can be one of two values. If an initialValue
was provided in the call to reduceRight
, then previousValue
will be equal to initialValue
and currentValue
will be equal to the last value in the array. If no initialValue
was provided, then previousValue
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(previousValue, currentValue, index, array) { return previousValue + currentValue; });
The callback would be invoked four times, with the arguments and return values in each call being as follows:
callback |
previousValue |
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(previousValue, currentValue, index, array) { return previousValue + currentValue; }, 10);
callback |
previousValue |
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
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]
Difference 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"
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; }; }
Specifications
Specification | Status | Comment |
---|---|---|
ECMAScript 5.1 (ECMA-262) The definition of 'Array.prototype.reduceRight' in that specification. |
Standard | Initial definition. Implemented in JavaScript 1.8. |
ECMAScript 2015 (6th Edition, ECMA-262) The definition of 'Array.prototype.reduceRight' in that specification. |
Standard | |
ECMAScript Latest Draft (ECMA-262) The definition of 'Array.prototype.reduceRight' in that specification. |
Draft |
Browser compatibility
Feature | Chrome | Edge | Firefox (Gecko) | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|---|
Basic support | (Yes) | (Yes) | 3.0 (1.9) | 9 | 10.5 | 4.0 |
Feature | Android | Chrome for Android | Edge | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile |
---|---|---|---|---|---|---|---|
Basic support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |