reducer

A reducer is a device or concept that decreases a quantity or transforms it to a different domain. The term is used in several fields, including computer science, plumbing, and mechanical engineering.

In computer science and programming, a reducer refers to a function that processes a sequence of values to produce a single accumulated result. In functional programming, reduce (or fold) applies a binary function to an initial accumulator and each element in turn. In JavaScript, the Array.prototype.reduce method implements this pattern. In Redux and similar architectures, a reducer is a pure function that takes a previous application state and an action, and returns a new state; it must not mutate inputs and should be deterministic.

In plumbing and piping, a reducer is a fitting that connects pipes of different diameters, narrowing the bore to adapt flow between sections. Reducers can be straight, or stepped, and are made from materials suitable for the fluid, pressure, and temperature involved, such as brass, stainless steel, or plastic.

In mechanical engineering, a gear reducer, or speed reducer, is a gearbox designed to decrease rotational speed from a motor or drive shaft while increasing torque. These devices are common in machinery to achieve workable output speeds and can be configured in inline or right-angle layouts with various gear types, including spur, helical, and planetary gear trains.