Javascript reduce array of objects

Try it

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

Parameters

callbackFn

A "reducer" function called with the following arguments:

The value resulting from the previous call to callbackFn. On first call, initialValue if specified, otherwise the value of array[0].

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].

The index position of currentValue in the array. On first call, 0 if initialValue was specified, otherwise 1.

The array being traversed.

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.

reduce() is a central concept in functional programming, where it's not possible to mutate any value, so in order to accumulate all values in an array, one must return a new accumulator value on every iteration. This convention propagates to JavaScript's reduce(): you should use spreading or other copying methods where possible to create new arrays and objects as the accumulator, rather than mutating the existing one. If you decided to mutate the accumulator instead of copying it, remember to still return the modified object in the callback, or the next iteration will receive undefined.

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(previousValue, currentValue, index) {
  const returns = previousValue + currentValue;
  console.log(
    `previousValue: ${previousValue}, currentValue: ${currentValue}, 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:

The array parameter never changes through the process — it's always [15, 16, 17, 18, 19]. 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) => previousValue + currentValue,
  10,
);

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

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

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.

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

console.log(sum); // logs 6

Flatten an array of arrays

const flattened = [
  [0, 1],
  [2, 3],
  [4, 5],
].reduce(
  (previousValue, currentValue) => previousValue.concat(currentValue),
  [],
);
// flattened is [0, 1, 2, 3, 4, 5]

Counting instances of values in an object

const names = ["Alice", "Bob", "Tiff", "Bruce", "Alice"];

const countedNames = names.reduce((allNames, name) => {
  const currCount = allNames[name] ?? 0;
  return {
    ...allNames,
    [name]: currCount + 1,
  };
}, {});
// 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((acc, obj) => {
    const key = obj[property];
    const curGroup = acc[key] ?? [];

    return { ...acc, [key]: [...curGroup, obj] };
  }, {});
}

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

Concatenating arrays contained in an array of objects using the spread syntax 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
const allbooks = friends.reduce(
  (previousValue, currentValue) => [...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: The same effect can be achieved with Set and Array.from() as const arrayWithNoDuplicates = Array.from(new Set(myArray)) with better performance.

const myArray = ["a", "b", "a", "b", "c", "e", "e", "c", "d", "d", "d", "d"];
const myArrayWithNoDuplicates = myArray.reduce(
  (previousValue, currentValue) => {
    if (!previousValue.includes(currentValue)) {
      return [...previousValue, currentValue];
    }
    return previousValue;
  },
  [],
);

console.log(myArrayWithNoDuplicates);

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

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

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

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

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

Running Promises in Sequence

/**
 * Chain a series of promise handlers.
 *
 * @param {array} arr - A list of promise handlers, each one receiving the
 * resolved result of the previous handler and returning another promise.
 * @param {*} input The initial value to start the promise chain
 * @return {Object} Final promise with a chain of handlers attached
 */
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) => 2 * 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

See also