It's pretty simple, the middleware function checks the action type, of the action being dispatched, to see if it exists as a property of the TYPE object constant. If it exists, then the middleware passes control to the next middleware in the chain. However, in our case, there is no next middleware, so the control is passed to the original dispatch method of the store that will apply the reducer and transform the state. On the other side, if the action type was not defined, the middleware function interrupts the middleware chain by not calling the next function and by throwing an error.