Summary
Apply a function simultaneously against two values of the array (from right-to-left) as to reduce it to a single value.
Method of Array |
|
---|---|
Implemented in | JavaScript 1.8 |
ECMAScript Edition | ECMAScript 5th Edition |
Syntax
array.reduceRight(callback[, initialValue])
Parameters
callback
- Function to execute on each value in the array.
initialValue
- Object to use as the first argument to the first call of the
callback
.
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.
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; }); // First call previousValue = 4, currentValue = 3, index = 3 // Second call previousValue = 7, currentValue = 2, index = 2 // Third call previousValue = 9, currentValue = 1, index = 1 // Fourth call previousValue = 10, currentValue = 0, index = 0 // array is always the object [0,1,2,3,4] upon which reduceRight was called // Return Value: 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); // First call previousValue = 10, currentValue = 4, index = 4 // Second call previousValue = 14, currentValue = 3, index = 3 // Third call previousValue = 17, currentValue = 2, index = 2 // Fourth call previousValue = 19, currentValue = 1, index = 1 // Fifth call previousValue = 20, currentValue = 0, index = 0 // array is always the object [0,1,2,3,4] upon which reduceRight was called // Return Value: 20
Compatibility
reduceRight
is a recent addition to the ECMA-262 standard; as such it may not be present in other 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. This algorithm is exactly the one specified in ECMA-262, 5th edition, assuming Object
and TypeError
have their original value and that callbackfn.call
evaluates to the original value of Function.prototype.call
.
if (!Array.prototype.reduceRight) { Array.prototype.reduceRight = function(callbackfn /*, initialValue */) { "use strict"; if (this == null) throw new TypeError(); var t = Object(this); var len = t.length >>> 0; if (typeof callbackfn != "function") throw new TypeError(); // no value to return if no initial value, empty array if (len === 0 && arguments.length === 1) throw new TypeError(); var k = len - 1; var accumulator; if (arguments.length >= 2) { accumulator = arguments[1]; } else { do { if (k in this) { accumulator = this[k--]; break; } // if array contains no values, no initial value to return if (--k < 0) throw new TypeError(); } while (true); } while (k >= 0) { if (k in t) accumulator = callbackfn.call(undefined, accumulator, t[k], k, t); k--; } return accumulator; }; }
Examples
Example: Sum up all values within an array
var total = [0, 1, 2, 3].reduceRight(function(a, b) { return a + b; }); // total == 6
Example: 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]
Browser compatibility
Based on Kangax's compat tables
Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|
Basic support | (Yes) | 3.0 (1.9) | 9 | 10.5 | 4.0 |
Feature | Android | Chrome for Android | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile |
---|---|---|---|---|---|---|
Basic support | ? | ? | ? | ? | ? | ? |