Array reduce method

Summary

Apply a function against an accumulator and each value of the array (from left-to-right) as to reduce it to a single value.

Method of Array
Implemented in	JavaScript 1.8
ECMAScript Edition	ECMAScript 5th Edition

Syntax

array.reduce(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

Object to use as the first argument to the first call of the callback.

Description

reduce 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 first time the callback is called, previousValue and currentValue can be one of two values. If initialValue is provided in the call to reduce, then previousValue will be equal to initialValue and currentValue will be equal to the first value in the array. If no initialValue was provided, then previousValue will be equal to the first value in the array and currentValue will be equal to the second.

Suppose the following use of reduce occurred:

[0,1,2,3,4].reduce(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:

	previousValue	currentValue	index	array	return value
first call	0	1	1	[0,1,2,3,4]	1
second call	1	2	2	[0,1,2,3,4]	3
third call	3	3	3	[0,1,2,3,4]	6
fourth call	6	4	4	[0,1,2,3,4]	10

The value returned by reduce would be that of the last callback invocation (10).

If you were to provide an initial value as the second argument to reduce, the result would look like this:

[0,1,2,3,4].reduce(function(previousValue, currentValue, index, array){
  return previousValue + currentValue;
}, 10);

	previousValue	currentValue	index	array	return value
first call	10	0	0	[0,1,2,3,4]	10
second call	10	1	1	[0,1,2,3,4]	11
third call	11	2	2	[0,1,2,3,4]	13
fourth call	13	3	3	[0,1,2,3,4]	16
fifth call	16	4	4	[0,1,2,3,4]	20

The value returned by reduce this time would be, of course, 20.

Compatibility

Array.prototype.reduce is an ECMAScript 5 addition; 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 reduce in implementations which do not natively support it.

if (!Array.prototype.reduce) {
  Array.prototype.reduce = function reduce(accumulator){
    if (this===null || this===undefined) throw new TypeError("Object is null or undefined");
    var i = 0, l = this.length >> 0, curr;

    if(typeof accumulator !== "function") // ES5 : "If IsCallable(callbackfn) is false, throw a TypeError exception."
      throw new TypeError("First argument is not callable");

    if(arguments.length < 2) {
      if (l === 0) throw new TypeError("Array length is 0 and no second argument");
      curr = this[0];
      i = 1; // start accumulating at the second element
    }
    else
      curr = arguments[1];

    while (i < l) {
      if(i in this) curr = accumulator.call(undefined, curr, this[i], i, this);
      ++i;
    }

    return curr;
  };
}

Examples

Example: Sum up all values within an array

var total = [0, 1, 2, 3].reduce(function(a, b) {
    return a + b;
});
// total == 6

Example: Flatten an array of arrays

var flattened = [[0, 1], [2, 3], [4, 5]].reduce(function(a, b) {
    return a.concat(b);
});
// flattened is [0, 1, 2, 3, 4, 5]

Browser compatibility

Based on Kangax's compat tables

See also

• reduceRight