reduce

INFO

This function is only available in es-toolkit/compat for compatibility reasons. It either has alternative native JavaScript APIs or isn’t fully optimized yet.

When imported from es-toolkit/compat, it behaves exactly like lodash and provides the same functionalities, as detailed here.

Reduces an array or object to a single value using an iteratee function

The reduce() function goes through each element in an array or values of an object and applies a special function (called a "reducer") to them, one by one. This function takes the result of the previous step and the current element to perform a calculation. After going through all the elements, the function gives you one final result.

When the reduce() function starts, there's no previous result to use. If you provide an initial value, it starts with that. If not, it uses the first element of the array or the first value of the object and begins with the second element or value for the calculation.

Signature

function reduce<T, U>(
  collection: T[],
  iteratee: (accumulator: U, value: T, index: number, collection: T[]) => U,
  initialValue: U
): U;
function reduce<T>(collection: T[], iteratee: (accumulator: T, value: T, index: number, collection: T[]) => T): T;

function reduce<T, U>(
  collection: ArrayLike<T>,
  iteratee: (accumulator: U, value: T, index: number, collection: ArrayLike<T>) => U,
  initialValue: U
): U;
function reduce<T>(
  collection: ArrayLike<T>,
  iteratee: (accumulator: T, value: T, index: number, collection: ArrayLike<T>) => T
): T;

function reduce<T extends object, U>(
  collection: T,
  iteratee: (accumulator: U, value: T[keyof T], key: keyof T, collection: T) => U,
  initialValue: U
): U;
function reduce<T extends object>(
  collection: T,
  iteratee: (accumulator: T[keyof T], value: T[keyof T], key: keyof T, collection: T) => T[keyof T]
): T[keyof T];

Parameters

• collection (T[] | ArrayLike<T> | Record<PropertyKey, T>): The collection to iterate over.
• iteratee ((accumulator: U, value: T, index, collection) => any)): The function invoked per iteration.
• initialValue (U): The initial value.

Returns

(any): Returns the accumulated value.

Examples

// Using a reducer function
const array = [1, 2, 3];
reduce(array, (acc, value) => acc + value, 0); // => 6

// Using a reducer function with initialValue
const array = [1, 2, 3];
reduce(array, (acc, value) => acc + value % 2 === 0, true); // => false

// Using an object as the collection
const obj = { a: 1, b: 2, c: 3 };
reduce(obj, (acc, value) => acc + value, 0); // => 6