Array.prototype.reduce()

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.

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 of callbackFn. For the last invocation, the return value becomes the return value of reduce(). The function is called with the following arguments:

accumulator

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 reduce() was called upon.

initialValue Optional

A value to which accumulator is initialized the first time the callback is called. If initialValue is specified, callbackFn starts executing with the first value in the array as currentValue. If initialValue is not specified, accumulator is initialized to the first value in the array, and callbackFn starts executing with the second value in the array as currentValue. In this case, if the array is empty (so that there's no first value to return as accumulator), an error is thrown.

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

TypeError

The array contains no elements and initialValue is not provided.

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().

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.

reduce() does not mutate the array on which it is called, but the function provided as callbackFn can. Note, however, that the length of the array is saved before the first invocation of callbackFn. Therefore:

• callbackFn will not visit any elements added beyond the array's initial length when the call to reduce() began.
• Changes to already-visited indexes do not cause callbackFn to be invoked on them again.
• If an existing, yet-unvisited element of the array is changed by callbackFn, its value passed to the callbackFn will be the value at the time that element gets visited. Deleted elements are not visited.

The reduce() method is generic. It only expects the this value to have a length property and integer-keyed properties.

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.

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

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:

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).

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:

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

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

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

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

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

The reduce() method reads the length property of this and then accesses each integer index.

const arrayLike = {
  length: 3,
  0: 2,
  1: 3,
  2: 4,
};
console.log(Array.prototype.reduce.call(arrayLike, (x, y) => x + y));
// 9

• Polyfill of Array.prototype.reduce in core-js
• Array.prototype.reduceRight()