What order does middleware run in?

• quiz
• medium
• state
• 10 min
• state-middleware
• mark done

Middleware is the only part of a Redux store that is not a pure function of state and action, which is exactly why its ordering surprises people.

With applyMiddleware(logger, thunk), in what order does an action pass through them?

• A thunk then logger, because thunk is closer to the store
• B logger then thunk, then the reducer, then back out through thunk and logger
• C Both run in parallel
• D Only the last one runs

Your logger is declared before thunk and prints '[object Function]' for every async action. Why?

• A The logger is broken
• B Thunks are not actions
• C The logger sits outside thunk, so it sees the function before thunk has a chance to call it
• D Redux serialises actions before logging

What does a middleware that forgets to call next(action) do?

• A Nothing, next() is optional
• B Swallows the action: the reducer never sees it and no later middleware runs
• C Causes an infinite loop
• D Dispatches the action twice

Inside middleware, when should you call store.dispatch(action) instead of next(action)?

• A They are interchangeable
• B dispatch restarts the action at the top of the chain; next passes it on. Use dispatch only for a NEW action, never to forward the current one
• C dispatch is faster
• D next is deprecated

Related

• Reading: Middleware · Store · Actions
• Playbook: State management
• Agent Skill: state-middleware