Reduce

👨‍💼 The reduce method accumulates an array into a single value. It's the most powerful (and sometimes confusing) array method—and a cornerstone of functional programming.

const numbers = [1, 2, 3, 4]
const sum = numbers.reduce((acc, n) => acc + n, 0)
// 10

Parameters:

accumulator (acc) - The running total
current (n) - The current element
initial value (0) - Starting point

🐨 Open

index.ts

and:

Use reduce to sum all prices
Use reduce to find the most expensive product
Use reduce to count products by category

💰 Reduce syntax:

const result = array.reduce((accumulator, current) => newAccumulator, initial)

💰 Counting by category (immutable pattern using spread):

const counts = products.reduce(
	(acc, p) => ({
		...acc,
		[p.category]: (acc[p.category] || 0) + 1,
	}),
	{} as Record<string, number>,
)

Each iteration returns a new object instead of mutating the accumulator. This is the functional approach—easier to reason about and debug.

📜 MDN - Array.prototype.reduce()

06. Array Methods/03. Reduce (💪 problem)

Reduce