Array.prototype.reduce()
Baseline Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
The reduce()
method of Array
instances 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).
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Syntax
reduce(callbackFn)
reduce(callbackFn, initialValue)
Parameters
callbackFn
-
A function to execute for each element in the array. Its return value becomes the value of the
accumulator
parameter on the next invocation ofcallbackFn
. For the last invocation, the return value becomes the return value ofreduce()
. The function is called with the following arguments:accumulator
-
The value resulting from the previous call to
callbackFn
. On the first call, its value isinitialValue
if the latter is specified; otherwise its value isarray[0]
. currentValue
-
The value of the current element. On the first call, its value is
array[0]
ifinitialValue
is specified; otherwise its value isarray[1]
. currentIndex
-
The index position of
currentValue
in the array. On the first call, its value is0
ifinitialValue
is specified, otherwise1
. array
-
The array
reduce()
was called upon.
initialValue
Optional-
A value to which
accumulator
is initialized the first time the callback is called. IfinitialValue
is specified,callbackFn
starts executing with the first value in the array ascurrentValue
. IfinitialValue
is not specified,accumulator
is initialized to the first value in the array, andcallbackFn
starts executing with the second value in the array ascurrentValue
. In this case, if the array is empty (so that there's no first value to return asaccumulator
), an error is thrown.
Return value
The value that results from running the "reducer" callback function to completion over the entire array.
Exceptions
TypeError
-
Thrown if the array contains no elements and
initialValue
is not provided.
Description
The reduce()
method is an iterative method. It runs a "reducer" callback function over all elements in the array, in ascending-index order, and accumulates them into a single value. Every time, the return value of callbackFn
is passed into callbackFn
again on next invocation as accumulator
. The final value of accumulator
(which is the value returned from callbackFn
on the final iteration of the array) becomes the return value of reduce()
. Read the iterative methods section for more information about how these methods work in general.
callbackFn
is invoked only for array indexes which have assigned values. It is not invoked for empty slots in sparse arrays.
Unlike other iterative methods, reduce()
does not accept a thisArg
argument. callbackFn
is always called with undefined
as this
, which gets substituted with globalThis
if callbackFn
is non-strict.
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. However, note that copying the accumulator may in turn lead to increased memory usage and degraded performance — see When to not use reduce() for more details. In such cases, to avoid bad performance and unreadable code, it's better to use a simple for
loop instead.
The reduce()
method is generic. It only expects the this
value to have a length
property and integer-keyed properties.
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(accumulator, currentValue, index) {
const returns = accumulator + currentValue;
console.log(
`accumulator: ${accumulator}, 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:
accumulator |
currentValue |
index |
Return value | |
---|---|---|---|---|
First call | 15 |
16 |
1 |
31 |
Second call | 31 |
17 |
2 |
48 |
Third call | 48 |
18 |
3 |
66 |
Fourth call | 66 |
19 |
4 |
85 |
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 as the second argument to reduce()
:
[15, 16, 17, 18, 19].reduce(
(accumulator, currentValue) => accumulator + currentValue,
10,
);
The callback would be invoked five times, with the arguments and return values in each call being as follows:
accumulator |
currentValue |
index |
Return value | |
---|---|---|---|---|
First call | 10 |
15 |
0 |
25 |
Second call | 25 |
16 |
1 |
41 |
Third call | 41 |
17 |
2 |
58 |
Fourth call | 58 |
18 |
3 |
76 |
Fifth call | 76 |
19 |
4 |
95 |
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(
(accumulator, currentValue) => accumulator + currentValue.x,
0,
);
console.log(sum); // 6
Function sequential piping
The pipe
function takes a sequence of functions and returns a new function. When the new function is called with an argument, the sequence of functions are called in order, which each one receiving the return value of the previous function.
const pipe =
(...functions) =>
(initialValue) =>
functions.reduce((acc, fn) => fn(acc), initialValue);
// Building blocks to use for composition
const double = (x) => 2 * x;
const triple = (x) => 3 * x;
const quadruple = (x) => 4 * x;
// 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
Running promises in sequence
Promise sequencing is essentially function piping demonstrated in the previous section, except done asynchronously.
// Compare this with pipe: fn(acc) is changed to acc.then(fn),
// and initialValue is ensured to be a promise
const asyncPipe =
(...functions) =>
(initialValue) =>
functions.reduce((acc, fn) => acc.then(fn), Promise.resolve(initialValue));
// Building blocks to use for composition
const p1 = async (a) => a * 5;
const p2 = async (a) => a * 2;
// The composed functions can also return non-promises, because the values are
// all eventually wrapped in promises
const f3 = (a) => a * 3;
const p4 = async (a) => a * 4;
asyncPipe(p1, p2, f3, p4)(10).then(console.log); // 1200
asyncPipe
can also be implemented using async
/await
, which better demonstrates its similarity with pipe
:
const asyncPipe =
(...functions) =>
(initialValue) =>
functions.reduce(async (acc, fn) => fn(await acc), initialValue);
Using reduce() with sparse arrays
reduce()
skips missing elements in sparse arrays, but it does not skip undefined
values.
console.log([1, 2, , 4].reduce((a, b) => a + b)); // 7
console.log([1, 2, undefined, 4].reduce((a, b) => a + b)); // NaN
Calling reduce() on non-array objects
The reduce()
method reads the length
property of this
and then accesses each property whose key is a nonnegative integer less than length
.
const arrayLike = {
length: 3,
0: 2,
1: 3,
2: 4,
3: 99, // ignored by reduce() since length is 3
};
console.log(Array.prototype.reduce.call(arrayLike, (x, y) => x + y));
// 9
When to not use reduce()
Multipurpose higher-order 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()
.
Note that reduce()
is always equivalent to a for...of
loop, except that instead of mutating a variable in the upper scope, we now return the new value for each iteration:
const val = array.reduce((acc, cur) => update(acc, cur), initialValue);
// Is equivalent to:
let val = initialValue;
for (const cur of array) {
val = update(val, cur);
}
As previously stated, the reason why people may want to use reduce()
is to mimic functional programming practices of immutable data. Therefore, developers who uphold the immutability of the accumulator often copy the entire accumulator for each iteration, like this:
const names = ["Alice", "Bob", "Tiff", "Bruce", "Alice"];
const countedNames = names.reduce((allNames, name) => {
const currCount = Object.hasOwn(allNames, name) ? allNames[name] : 0;
return {
...allNames,
[name]: currCount + 1,
};
}, {});
This code is ill-performing, because each iteration has to copy the entire allNames
object, which could be big, depending how many unique names there are. This code has worst-case O(N^2)
performance, where N
is the length of names
.
A better alternative is to mutate the allNames
object on each iteration. However, if allNames
gets mutated anyway, you may want to convert the reduce()
to a simple for
loop instead, which is much clearer:
const names = ["Alice", "Bob", "Tiff", "Bruce", "Alice"];
const countedNames = names.reduce((allNames, name) => {
const currCount = allNames[name] ?? 0;
allNames[name] = currCount + 1;
// return allNames, otherwise the next iteration receives undefined
return allNames;
}, Object.create(null));
const names = ["Alice", "Bob", "Tiff", "Bruce", "Alice"];
const countedNames = Object.create(null);
for (const name of names) {
const currCount = countedNames[name] ?? 0;
countedNames[name] = currCount + 1;
}
Therefore, if your accumulator is an array or an object and you are copying the array or object on each iteration, you may accidentally introduce quadratic complexity into your code, causing performance to quickly degrade on large data. This has happened in real-world code — see for example Making Tanstack Table 1000x faster with a 1 line change.
Some of the acceptable use cases of reduce()
are given above (most notably, summing an array, promise sequencing, and function piping). There are other cases where better alternatives than reduce()
exist.
- Flattening an array of arrays. Use
flat()
instead.jsconst flattened = array.reduce((acc, cur) => acc.concat(cur), []);
jsconst flattened = array.flat();
- Grouping objects by a property. Use
Object.groupBy()
instead.jsconst groups = array.reduce((acc, obj) => { const key = obj.name; const curGroup = acc[key] ?? []; return { ...acc, [key]: [...curGroup, obj] }; }, {});
jsconst groups = Object.groupBy(array, (obj) => obj.name);
- Concatenating arrays contained in an array of objects. Use
flatMap()
instead.jsconst friends = [ { name: "Anna", books: ["Bible", "Harry Potter"] }, { name: "Bob", books: ["War and peace", "Romeo and Juliet"] }, { name: "Alice", books: ["The Lord of the Rings", "The Shining"] }, ]; const allBooks = friends.reduce((acc, cur) => [...acc, ...cur.books], []);
jsconst allBooks = friends.flatMap((person) => person.books);
- Removing duplicate items in an array. Use
Set
andArray.from()
instead.jsconst uniqArray = array.reduce( (acc, cur) => (acc.includes(cur) ? acc : [...acc, cur]), [], );
jsconst uniqArray = Array.from(new Set(array));
- Eliminating or adding elements in an array. Use
flatMap()
instead.js// Takes an array of numbers and splits perfect squares into its square roots const roots = array.reduce((acc, cur) => { if (cur < 0) return acc; const root = Math.sqrt(cur); if (Number.isInteger(root)) return [...acc, root, root]; return [...acc, cur]; }, []);
If you are only eliminating elements from an array, you also can usejsconst roots = array.flatMap((val) => { if (val < 0) return []; const root = Math.sqrt(val); if (Number.isInteger(root)) return [root, root]; return [val]; });
filter()
. - Searching for elements or testing if elements satisfy a condition. Use
find()
andfindIndex()
, orsome()
andevery()
instead. These methods have the additional benefit that they return as soon as the result is certain, without iterating the entire array.jsconst allEven = array.reduce((acc, cur) => acc && cur % 2 === 0, true);
jsconst allEven = array.every((val) => val % 2 === 0);
In cases where reduce()
is the best choice, documentation and semantic variable naming can help mitigate readability drawbacks.
Specifications
Specification |
---|
ECMAScript Language Specification # sec-array.prototype.reduce |
Browser compatibility
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