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((sum, number) => sum + number, 0);
// x == 6

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