The 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 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.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. reduce[]
is to return the sum of all the elements in an array:Try it
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:
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 being traversed.
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.
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 byreduce[]
.
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
Specifications
ECMAScript Language Specification # sec-array.prototype.reduce |
Browser compatibility
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