The reduce() method executes a reducer function (that you provide) on each member of the array resulting in a single output value.

The reducer function is fed four parameters:

  1. Accumulator (acc)
  2. Current Value (cur)
  3. Current Index (idx)
  4. Source Array (src)

Your reducer function's returned value is assigned to the accumulator, whose value is remembered across each iteration throughout the array and ultimately becomes the final, single resulting value.

Syntax

arr.reduce(callback[, initialValue])

Parameters

callback
Function to execute on each element in the array, taking four arguments:
accumulator
The accumulator accumulates the callback's return values; it is the accumulated 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.
currentIndexOptional
The index of the current element being processed in the array. Starts at index 0, if an initialValue is provided, and at index 1 otherwise.
arrayOptional
The array reduce() was called upon.
initialValueOptional
Value to use as the first argument to the first call of the callback. If no initial value is supplied, the first element in the array will be used. Calling reduce() on an empty array without an initial value is an error.

Return value

The value that results from the reduction.

Description

reduce() executes the callback function once for each element present in the array, excluding holes in the array, receiving four arguments:

  • accumulator
  • currentValue
  • currentIndex
  • array

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

Note: If initialValue isn't provided, reduce() will execute the callback function starting at index 1, skipping the first index. If initialValue is provided, it will start at index 0.

If the array is empty and no initialValue is provided, TypeError will be thrown. If the array has only one element (regardless of position) and no initialValue is provided, or if initialValue is provided but the array is empty, the solo value will be returned without calling callback.

It is usually safer to provide an initial value because there are three possible outputs without initialValue, as shown in the following example.

var maxCallback = ( acc, cur ) => Math.max( acc.x, cur.x );
var maxCallback2 = ( max, cur ) => Math.max( max, cur );

// reduce() without initialValue
[ { x: 22 }, { x: 42 } ].reduce( maxCallback ); // 42
[ { x: 22 }            ].reduce( maxCallback ); // { x: 22 }
[                      ].reduce( maxCallback ); // TypeError

// map/reduce; better solution, also works for empty or larger arrays
[ { x: 22 }, { x: 42 } ].map( el => el.x )
                        .reduce( maxCallback2, -Infinity );

How reduce() works

Suppose the following use of reduce() occurred:

[0, 1, 2, 3, 4].reduce(function(accumulator, currentValue, currentIndex, 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 currentIndex 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).

You can also provide an Arrow Function in lieu of a full function. The code below will produce the same output as the code in the block above:

[0, 1, 2, 3, 4].reduce( (accumulator, currentValue, currentIndex, array) => accumulator + currentValue );

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((accumulator, currentValue, currentIndex, array) => {
    return accumulator + currentValue;
}, 10);
callback accumulator currentValue currentIndex 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() in this case would be 20.

Examples

Sum all the values of an array

var sum = [0, 1, 2, 3].reduce(function (accumulator, currentValue) {
  return accumulator + currentValue;
}, 0);
// sum is 6

Alternatively, written with an arrow function:

var total = [ 0, 1, 2, 3 ].reduce(
  ( accumulator, currentValue ) => accumulator + currentValue,
  0
);

Sum of values in an object array

To sum up values contained in an array of objects you must supply an initial value so that each item passes through your function.

var initialValue = 0;
var sum = [{x: 1}, {x:2}, {x:3}].reduce(function (accumulator, currentValue) {
    return accumulator + currentValue.x;
},initialValue)

console.log(sum) // logs 6

Alternatively, written with an arrow function: 

var initialValue = 0;
var sum = [{x: 1}, {x:2}, {x:3}].reduce(
    (accumulator, currentValue) => accumulator + currentValue.x
    ,initialValue
);

console.log(sum) // logs 6

Flatten an array of arrays

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

Alternatively, written with an arrow function:

var flattened = [[0, 1], [2, 3], [4, 5]].reduce(
  ( accumulator, currentValue ) => accumulator.concat(currentValue),
  []
);

Counting instances of values in an object

var names = ['Alice', 'Bob', 'Tiff', 'Bruce', 'Alice'];

var countedNames = names.reduce(function (allNames, name) { 
  if (name in allNames) {
    allNames[name]++;
  }
  else {
    allNames[name] = 1;
  }
  return allNames;
}, {});
// countedNames is:
// { 'Alice': 2, 'Bob': 1, 'Tiff': 1, 'Bruce': 1 }

Grouping objects by a property

var people = [
  { name: 'Alice', age: 21 },
  { name: 'Max', age: 20 },
  { name: 'Jane', age: 20 }
];

function groupBy(objectArray, property) {
  return objectArray.reduce(function (acc, obj) {
    var key = obj[property];
    if (!acc[key]) {
      acc[key] = [];
    }
    acc[key].push(obj);
    return acc;
  }, {});
}

var groupedPeople = groupBy(people, 'age');
// groupedPeople is:
// { 
//   20: [
//     { name: 'Max', age: 20 }, 
//     { name: 'Jane', age: 20 }
//   ], 
//   21: [{ name: 'Alice', age: 21 }] 
// }

Bonding arrays contained in an array of objects using the spread operator and initialValue

// friends - an array of objects 
// where object field "books" - list of favorite books 
var friends = [{
  name: 'Anna',
  books: ['Bible', 'Harry Potter'],
  age: 21
}, {
  name: 'Bob',
  books: ['War and peace', 'Romeo and Juliet'],
  age: 26
}, {
  name: 'Alice',
  books: ['The Lord of the Rings', 'The Shining'],
  age: 18
}];

// allbooks - list which will contain all friends' books +  
// additional list contained in initialValue
var allbooks = friends.reduce(function(accumulator, currentValue) {
  return [...accumulator, ...currentValue.books];
}, ['Alphabet']);

// allbooks = [
//   'Alphabet', 'Bible', 'Harry Potter', 'War and peace', 
//   'Romeo and Juliet', 'The Lord of the Rings',
//   'The Shining'
// ]

 

Remove duplicate items in array

let arr = [1, 2, 1, 2, 3, 5, 4, 5, 3, 4, 4, 4, 4];
let result = arr.sort().reduce((accumulator, current) => {
    const length = accumulator.length
    if (length === 0 || accumulator[length - 1] !== current) {
        accumulator.push(current);
    }
    return accumulator;
}, []);
console.log(result); //[1,2,3,4,5]

Running Promises in Sequence

/**
 * Runs promises from array of functions that can return promises
 * in chained manner
 *
 * @param {array} arr - promise arr
 * @return {Object} promise object
 */
function runPromiseInSequence(arr, input) {
  return arr.reduce(
    (promiseChain, currentFunction) => promiseChain.then(currentFunction),
    Promise.resolve(input)
  );
}

// promise function 1
function p1(a) {
  return new Promise((resolve, reject) => {
    resolve(a * 5);
  });
}

// promise function 2
function p2(a) {
  return new Promise((resolve, reject) => {
    resolve(a * 2);
  });
}

// function 3  - will be wrapped in a resolved promise by .then()
function f3(a) {
 return a * 3;
}

// promise function 4
function p4(a) {
  return new Promise((resolve, reject) => {
    resolve(a * 4);
  });
}

const promiseArr = [p1, p2, f3, p4];
runPromiseInSequence(promiseArr, 10)
  .then(console.log);   // 1200

Function composition enabling piping

// Building-blocks to use for composition
const double = x => x + x;
const triple = x => 3 * x;
const quadruple = x => 4 * x;

// Function composition enabling pipe functionality
const pipe = (...functions) => input => functions.reduce(
    (acc, fn) => fn(acc),
    input
);

// Composed functions for multiplication of specific values
const multiply6 = pipe(double, triple);
const multiply9 = pipe(triple, triple);
const multiply16 = pipe(quadruple, quadruple);
const multiply24 = pipe(double, triple, quadruple);

// Usage
multiply6(6); // 36
multiply9(9); // 81
multiply16(16); // 256
multiply24(10); // 240

write map using reduce

if (!Array.prototype.mapUsingReduce) {
  Array.prototype.mapUsingReduce = function(callback, thisArg) {
    return this.reduce(function(mappedArray, currentValue, index, array) {
      mappedArray[index] = callback.call(thisArg, currentValue, index, array);
      return mappedArray;
    }, []);
  };
}

[1, 2, , 3].mapUsingReduce(
  (currentValue, index, array) => currentValue + index + array.length
); // [5, 7, , 10]

Polyfill

// Production steps of ECMA-262, Edition 5, 15.4.4.21
// Reference: http://es5.github.io/#x15.4.4.21
// https://tc39.github.io/ecma262/#sec-array.prototype.reduce
if (!Array.prototype.reduce) {
  Object.defineProperty(Array.prototype, 'reduce', {
    value: function(callback /*, initialValue*/) {
      if (this === null) {
        throw new TypeError( 'Array.prototype.reduce ' + 
          'called on null or undefined' );
      }
      if (typeof callback !== 'function') {
        throw new TypeError( callback +
          ' is not a function');
      }

      // 1. Let O be ? ToObject(this value).
      var o = Object(this);

      // 2. Let len be ? ToLength(? Get(O, "length")).
      var len = o.length >>> 0; 

      // Steps 3, 4, 5, 6, 7      
      var k = 0; 
      var value;

      if (arguments.length >= 2) {
        value = arguments[1];
      } else {
        while (k < len && !(k in o)) {
          k++; 
        }

        // 3. If len is 0 and initialValue is not present,
        //    throw a TypeError exception.
        if (k >= len) {
          throw new TypeError( 'Reduce of empty array ' +
            'with no initial value' );
        }
        value = o[k++];
      }

      // 8. Repeat, while k < len
      while (k < len) {
        // a. Let Pk be ! ToString(k).
        // b. Let kPresent be ? HasProperty(O, Pk).
        // c. If kPresent is true, then
        //    i.  Let kValue be ? Get(O, Pk).
        //    ii. Let accumulator be ? Call(
        //          callbackfn, undefined,
        //          « accumulator, kValue, k, O »).
        if (k in o) {
          value = callback(value, o[k], k, o);
        }

        // d. Increase k by 1.      
        k++;
      }

      // 9. Return accumulator.
      return value;
    }
  });
}

If you need to support truly obsolete JavaScript engines that don't support Object.defineProperty(), it's best not to polyfill Array.prototype methods at all, as you can't make them non-enumerable.

Specifications

Specification Status Comment
ECMAScript 5.1 (ECMA-262)
The definition of 'Array.prototype.reduce()' in that specification.
Standard Initial definition. Implemented in JavaScript 1.8.
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Array.prototype.reduce()' in that specification.
Standard  
ECMAScript Latest Draft (ECMA-262)
The definition of 'Array.prototype.reduce()' in that specification.
Draft  

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support Yes Yes3910.54
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidOpera AndroidiOS SafariSamsung Internet
Basic support Yes Yes Yes4 Yes Yes Yes

See also