redux/src/types/middleware.ts

import { Dispatch } from './store'

export interface MiddlewareAPI<D extends Dispatch = Dispatch, S = any> {
  dispatch: D
  getState(): S
}

/**
 * A middleware is a higher-order function that composes a dispatch function
 * to return a new dispatch function. It often turns async actions into
 * actions.
 *
 * Middleware is composable using function composition. It is useful for
 * logging actions, performing side effects like routing, or turning an
 * asynchronous API call into a series of synchronous actions.
 *
 * @template DispatchExt Extra Dispatch signature added by this middleware.
 * @template S The type of the state supported by this middleware.
 * @template D The type of Dispatch of the store where this middleware is
 *   installed.
 */
export interface Middleware<
  _DispatchExt = {}, // TODO: see if this can be used in type definition somehow (can't be removed, as is used to get final dispatch type)
  S = any,
  D extends Dispatch = Dispatch
> {
  (api: MiddlewareAPI<D, S>): (
    next: (action: unknown) => unknown
  ) => (action: unknown) => unknown
}