What is the Redux middleware signature?

Middleware is written as three nested (curried) functions:

const exampleMiddleware = (store) => (next) => (action) => {
// store  => { getState, dispatch }
// next   => calls the next middleware in the chain (or the reducer if last)
// action => the action currently being dispatched
return next(action);
};

• store gives access to getState() and dispatch().
• next passes the action along to the next middleware (or the root reducer, if this is the last middleware).
• action is the action object being processed.

Middleware must call next(action) (or dispatch(...) for a different action) to keep the chain moving — forgetting to call next silently swallows every action.