Array.prototype.reduce()

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

The reducer function takes four arguments:

1. Accumulator
2. Current Value
3. Current Index
4. Source Array

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

// Arrow function
reduce((accumulator, currentValue) => { ... } )
reduce((accumulator, currentValue, index) => { ... } )
reduce((accumulator, currentValue, index, array) => { ... } )
reduce((accumulator, currentValue, index, array) => { ... }, initialValue)

// Reducer function
reduce(reducerFn)
reduce(reducerFn, initialValue)

// Inline reducer function
reduce(function reducerFn(accumulator, currentValue) { ... })
reduce(function reducerFn(accumulator, currentValue, index) { ... })
reduce(function reducerFn(accumulator, currentValue, index, array){ ... })
reduce(function reducerFn(accumulator, currentValue, index, array) { ... }, initialValue)

Parameters

callback A function to execute on each element in the array (except for the first, if no initialValue is supplied).

It takes four arguments:

accumulator The accumulator accumulates callback's return values. It is the accumulated value previously returned in the last invocation of the callback—or initialValue, if it was supplied (see below).

currentValue The current element being processed in the array.

index Optional The index of the current element being processed in the array. Starts from index 0 if an initialValue is provided. Otherwise, it starts from index 1.

array Optional The array reduce() was called upon.

initialValue Optional A value to use as the first argument to the first call of the callback. If no initialValue is supplied, the first element in the array will be used as the initial accumulator value and skipped as currentValue. Calling reduce() on an empty array without an initialValue will throw a TypeError.

Return value

The single value that results from the reduction.

Description

The reduce() method executes the callback once for each assigned value present in the array, taking four arguments:

1. accumulator
2. currentValue
3. currentIndex
4. 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 is not 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 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 callback.

It is almost always safer to provide an initialValue, because there can be up to four possible output types without initialValue, as shown in the following example:

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

// reduce without initialValue
[ { x: 2 }, { x: 22 }, { x: 42 } ].reduce( maxCallback ); // NaN
[ { x: 2 }, { x: 22 }            ].reduce( maxCallback ); // 22
[ { x: 2 }                       ].reduce( maxCallback ); // { x: 2 }
[                                ].reduce( maxCallback ); // TypeError

// map & reduce with initialValue; 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:

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

You can also provide an Arrow Function instead 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 initialValue 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)

The value returned by reduce() in this case would be 20.

Polyfill

// Production steps of ECMA-262, Edition 5, 15.4.4.21
// Reference: https://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;
    }
  });
}

Note: If you need to support truly obsolete JavaScript engines that do not support Object.defineProperty(), it is best not to polyfill Array.prototype methods at all, as you cannot make them non-enumerable.

Examples

Sum all the values of an array

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

Alternatively written with an arrow function:

let total = [ 0, 1, 2, 3 ].reduce(
  ( accumulator, currentValue ) => accumulator + 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 (accumulator, currentValue) {
    return accumulator + 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(
    (accumulator, currentValue) => accumulator + currentValue.x
    , initialValue
)

console.log(sum) // logs 6

Flatten an array of arrays

let 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:

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

Counting instances of values in an object

let 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

let 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
let 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(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 an array

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

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

console.log(myOrderedArray)

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

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

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