Array.prototype.reduce()

The reduce() method executes a user-supplied "reducer" callback function on each element of the array, in order, passing in the return value from the calculation on the preceding element. The final result of running the reducer across all elements of the array is a single value.

The first time that the callback is run there is no "return value of the previous calculation". If supplied, an initial value may be used in its place. Otherwise the array element at index 0 is used as the initial value and iteration starts from the next element (index 1 instead of index 0).

Perhaps the easiest-to-understand case for reduce() is to return the sum of all the elements in an array:

The reducer walks through the array element-by-element, at each step adding the current array value to the result from the previous step (this result is the running sum of all the previous steps) — until there are no more elements to add.

Syntax

// Arrow function
reduce((previousValue, currentValue) => { /* ... */ } )
reduce((previousValue, currentValue, currentIndex) => { /* ... */ } )
reduce((previousValue, currentValue, currentIndex, array) => { /* ... */ } )
reduce((previousValue, currentValue, currentIndex, array) => { /* ... */ }, initialValue)

// Callback function
reduce(callbackFn)
reduce(callbackFn, initialValue)

// Inline callback function
reduce(function(previousValue, currentValue) { /* ... */ })
reduce(function(previousValue, currentValue, currentIndex) { /* ... */ })
reduce(function(previousValue, currentValue, currentIndex, array) { /* ... */ })
reduce(function(previousValue, currentValue, currentIndex, array) { /* ... */ }, initialValue)

Parameters

callbackFn

A "reducer" function.

The function is called with the following arguments:

previousValue: the value resulting from the previous call to callbackFn. On first call, initialValue if specified, otherwise the value of array[0].
currentValue: the value of the current element. On first call, the value of array[0] if an initialValue was specified, otherwise the value of array[1].
currentIndex: the index position of currentValue in the array. On first call, 0 if initialValue was specified, otherwise 1.
array: the array to traverse.

initialValue Optional

A value to which previousValue is initialized the first time the callback is called. If initialValue is specified, that also causes currentValue to be initialized to the first value in the array. If initialValue is not specified, previousValue is initialized to the first value in the array, and currentValue is initialized to the second value in the array.

Return value

The value that results from running the "reducer" callback function to completion over the entire array.

Exceptions

TypeError: The array contains no elements and initialValue is not provided.

Description

The reduce() method takes two arguments: a callback function and an optional initial value. If an initial value is provided, reduce() calls the "reducer" callback function on each element in the array, in order. If no initial value is provided, reduce() calls the callback function on each element in the array after the first element.

reduce() returns the value that is returned from the callback function on the final iteration of the array.

When to not use reduce()

Recursive functions like reduce() can be powerful but sometimes difficult to understand, especially for less experienced JavaScript developers. If code becomes clearer when using other array methods, developers must weigh the readability tradeoff against the other benefits of using reduce(). In cases where reduce() is the best choice, documentation and semantic variable naming can help mitigate readability drawbacks.

Behavior during array mutations

The reduce() method itself does not mutate the array it is used on. However, it is possible for code inside the callback function to mutate the array. These are the possible scenarios of array mutations and how reduce() behaves in these scenarios:

If elements are appended to the array after reduce() begins to iterate over the array, the callback function does not iterate over the appended elements.
If existing elements of the array do get changed, the values passed to the callback function will be the values from the time that reduce() was first called on the array.
Array elements that are deleted after the call to reduce() begins and before being iterated over are not visited by reduce().

Edge cases

If the array only has 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 callbackFn.

If initialValue is provided and the array is not empty, then the reduce method will always invoke the callback function starting at index 0.

If initialValue is not provided then the reduce method will act differently for arrays with length larger than 1, equal to 1 and 0, as shown in the following example:

const getMax = (a, b) => Math.max(a, b);

// callback is invoked for each element in the array starting at index 0
[1, 100].reduce(getMax, 50); // 100
[    50].reduce(getMax, 10); // 50

// callback is invoked once for element at index 1
[1, 100].reduce(getMax);     // 100

// callback is not invoked
[    50].reduce(getMax);     // 50
[      ].reduce(getMax, 1);  // 1

[      ].reduce(getMax);     // TypeError

Examples

How reduce() works without an initial value

The code below shows what happens if we call reduce() with an array and no initial value.

const array = [15, 16, 17, 18, 19];

function reducer(previous, current, index, array) {
  const returns = previous + current;
  console.log(`previous: ${previous}, current: ${current}, index: ${index}, returns: ${returns}`);
  return returns;
}

array.reduce(reducer);

The callback would be invoked four times, with the arguments and return values in each call being as follows:

`callback` iteration	`previousValue`	`currentValue`	`currentIndex`	`array`	return value
first call	`15`	`16`	`1`	`[15, 16, 17, 18, 19]`	`31`
second call	`31`	`17`	`2`	`[15, 16, 17, 18, 19]`	`48`
third call	`48`	`18`	`3`	`[15, 16, 17, 18, 19]`	`66`
fourth call	`66`	`19`	`4`	`[15, 16, 17, 18, 19]`	`85`

The value returned by reduce() would be that of the last callback invocation (85).

How reduce() works with an initial value

Here we reduce the same array using the same algorithm, but with an initialValue of 10 passed the second argument to reduce():

[15, 16, 17, 18, 19].reduce( (previousValue, currentValue, currentIndex, array) => previousValue + currentValue, 10 )

The callback would be invoked five times, with the arguments and return values in each call being as follows:

`callback` iteration	`previousValue`	`currentValue`	`currentIndex`	`array`	return value
first call	`10`	`15`	`0`	`[15, 16, 17, 18, 19]`	`25`
second call	`25`	`16`	`1`	`[15, 16, 17, 18, 19]`	`41`
third call	`41`	`17`	`2`	`[15, 16, 17, 18, 19]`	`58`
fourth call	`58`	`18`	`3`	`[15, 16, 17, 18, 19]`	`76`
fifth call	`76`	`19`	`4`	`[15, 16, 17, 18, 19]`	`95`

The value returned by reduce() in this case would be 95.

Sum all the values of an array

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

Alternatively written with an arrow function:

let total = [ 0, 1, 2, 3 ].reduce(
  ( previousValue, currentValue ) => previousValue + currentValue,
  0
)

Sum of values in an object array

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

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

console.log(sum) // logs 6

Alternatively written with an arrow function:

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

console.log(sum) // logs 6

Flatten an array of arrays

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

Alternatively written with an arrow function:

let flattened = [[0, 1], [2, 3], [4, 5]].reduce(
  ( previousValue, currentValue ) => previousValue.concat(currentValue),
  []
)

Counting instances of values in an object

const names = ['Alice', 'Bob', 'Tiff', 'Bruce', 'Alice']

let 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

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

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

let 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" is a list of favorite books
const 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
let allbooks = friends.reduce(function(previousValue, currentValue) {
  return [...previousValue, ...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 an array

Note: If you are using an environment compatible with Set and Array.from(), you could use let arrayWithNoDuplicates = Array.from(new Set(myArray)) to get an array where duplicate items have been removed.

const myArray = ['a', 'b', 'a', 'b', 'c', 'e', 'e', 'c', 'd', 'd', 'd', 'd']
let myArrayWithNoDuplicates = myArray.reduce(function (previousValue, currentValue) {
  if (previousValue.indexOf(currentValue) === -1) {
    previousValue.push(currentValue)
  }
  return previousValue
}, [])

console.log(myArrayWithNoDuplicates)

Replace .filter().map() with .reduce()

Using Array.filter() then Array.map() traverses the array twice, but you can achieve the same effect while traversing only once with Array.reduce(), thereby being more efficient. (If you like for loops, you can filter and map while traversing once with Array.forEach()).

const numbers = [-5, 6, 2, 0,];

const doubledPositiveNumbers = numbers.reduce((previousValue, currentValue) => {
  if (currentValue > 0) {
    const doubled = currentValue * 2;
    previousValue.push(doubled);
  }
  return previousValue;
}, []);

console.log(doubledPositiveNumbers); // [12, 4]

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) => initialValue => functions.reduce(
    (acc, fn) => fn(acc),
    initialValue
)

// 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, initialValue) {
    return this.reduce(function(mappedArray, currentValue, currentIndex, array) {
      mappedArray[currentIndex] = callback.call(initialValue, currentValue, currentIndex, array)
      return mappedArray
    }, [])
  }
}

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

Specifications

Specification
ECMAScript Language Specification # sec-array.prototype.reduce

Browser compatibility

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