Review of Reducers and Pure Functions

One of the core concepts of Redux is the reducer. A reducer is a function with the signature (accumulator: T, item: U) => T. Reducers are often used in JavaScript through the Array.reduce method, which iterates over each of the array's items and accumulates a single value as a result. Reducers should be pure functions, meaning they don't generate any side-effects.

A simple example of a reducer is the sum function:

let x = [1,2,3].reduce((value, state) => value + state, 0)
// x == 6