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reduce()

reduce<T, Accumulator>(collection, iteratee, accumulator): Accumulator

reduce<T>(collection, iteratee): T | undefined

reduce<T, Accumulator>(collection, iteratee, accumulator): Accumulator

Reduces collection to a value which is the accumulated result of running each element in collection thru iteratee.

DEPRECATED

Use array.reduce() or Object.values().reduce() directly instead.

Type Parameters​

T: T​

The type of elements in the array.

Accumulator: Accumulator​

The type of the accumulated value.

Parameters​

Overload 1:

collection: T[]​

The collection to iterate over.

iteratee: (accumulator, value, index, array) => Accumulator​

The function invoked per iteration.

accumulator: Accumulator​

The initial value.

Overload 2:

collection: T[]​

The collection to iterate over.

iteratee: (accumulator, value, index, array) => T​

The function invoked per iteration.

Overload 3:

collection: T​

The collection to iterate over.

iteratee: (accumulator, value, key, object) => Accumulator​

The function invoked per iteration.

accumulator: Accumulator​

The initial value.

Returns: Accumulator​

The accumulated value.

See Also​

Since​

2.0.0

Also known as​

reduce (Lodash, es-toolkit, Remeda, Ramda, Effect) · ❌ (Radashi, Modern Dash, Antfu)

Example​

const numbers = [1, 2, 3, 4, 5];

// ❌ Deprecated approach
const sum = reduce(numbers, (acc, num) => acc + num, 0);
console.log(sum); // 15

// ✅ Recommended approach (for arrays)
const sumNative = numbers.reduce((acc, num) => acc + num, 0);
console.log(sumNative); // 15

// ✅ Recommended approach (for objects)
const obj = { a: 1, b: 2, c: 3 };
const sumObj = Object.values(obj).reduce((acc, num) => acc + num, 0);
console.log(sumObj); // 6

How it works?​

Reduces collection to a value by iterating elements. Deprecated: Use array.reduce() directly.

Native Equivalent​

// ❌ reduce(arr, fn, init)
// ✅ arr.reduce(fn, init)

Use Cases​

Aggregate values into single result 📌​

Combine all elements into a single value.

const prices = [10, 20, 30, 40];
prices.reduce((sum, price) => sum + price, 0);
// => 100

Group items by key​

Categorize items into groups.

const items = [{ type: "a" }, { type: "b" }, { type: "a" }];
items.reduce((acc, item) => {
  (acc[item.type] ??= []).push(item);
  return acc;
}, {});

Build lookup map​

Create an object for fast lookups.

const users = [{ id: 1, name: "Alice" }, { id: 2, name: "Bob" }];
users.reduce((map, u) => ({ ...map, [u.id]: u }), {});
// => { 1: { id: 1, ... }, 2: { id: 2, ... } }

Compute multiple statistics in a single pass​

Calculate several metrics (sum, count, min, max) without iterating multiple times. Efficient for analytics, reporting, and data processing pipelines.

const orders = [
  { amount: 120 },
  { amount: 45 },
  { amount: 230 },
  { amount: 89 },
];

const stats = reduce(
  orders,
  (acc, order) => ({
    total: acc.total + order.amount,
    count: acc.count + 1,
    min: Math.min(acc.min, order.amount),
    max: Math.max(acc.max, order.amount),
  }),
  { total: 0, count: 0, min: Infinity, max: -Infinity }
);

const average = stats.total / stats.count;
// stats => { total: 484, count: 4, min: 45, max: 230 }
// average => 121