"React has detected a change in the order of Hooks" but Hooks seem to be invoked in order

Question

I am trying to use Context and Reducers via React's hooks, and running into problems with the order of the hooks not being constant. My understanding was that as long as the order of the useHook(…) remained the same, it was fine to invoke the returned state/update function/reducer in any sort of control flow. Otherwise, I'm invoking the hooks at the very beginning of the FunctionComponents.

Is it that I'm generating Days in a loop? Or missing something else?

Warning: React has detected a change in the order of Hooks called by Container. This will lead to bugs and errors if not fixed. For more information, read the Rules of Hooks: https://reactjs.org/docs/hooks-rules.html     Previous render            Next render    ------------------------------------------------------ 1. useContext                 useContext 2. undefined                  useRef    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 

The full version of Container is below. An excerpt from Day is below, and has a ref from react-dnd's useDrop.

export const Container: FunctionComponent<Props> = () => {   let events = useContext(State.StateContext)   //let events: Array<Event.Event> = [] <- with this, no warning    const getDaysEvents = (day: Event.Time, events: Array<Event.Event>) => {     return events.map(e => {       const isTodays = e.startTime.hasSame(day, "day")       return isTodays && Event.Event({ dayHeight, event: e })     })   }    let days = []   for (let i = 0; i < 7; i++) {     const day = DateTime.today().plus({ days: i })     days.push(       <Day key={day.toISO()} height={dayHeight} date={day}>         {getDaysEvents(day, events)}       </Day>     )   }   return <div className="Container">{days}</div> } 

An excerpt from Day (Event similarly uses a useDrag hook, also called at the top level just like here).

const Day: FunctionComponent<DayProps> = ({ date, height, children }) => {   const dispatch = useContext(State.DispatchContext)   const [{ isOver, offset }, dropRef] = useDrop({     // …uses the dispatch function within…     // …   })   // … } 

Answer 1

I ran into this same error message in a component I was writing due to use of short-circuiting logic.

This resulted in an error:

const x = useSelector(state => state.foo); if (!x) { return ; } const y = useSelector(state => state.bar);  

This is because when x is truthy the list of hooks has length 2, but when x is falsey the list has length 1.

To resolve the error I had to put all hook use before any early terminations.

const x = useSelector(state => state.foo); const y = useSelector(state => state.bar); if (!x) { return ; }  

Answer 2

Writing my comment as an answer:

The problem is that you're calling Event.Event() directly, even though it is a react component. That causes react to treat the hook calls inside the function as part of Container, even though you meant for them to be part of Event.

The solution is to use JSX:

return isTodays && <Event.Event dayHeight={dayHeight} event={e} />

Why this works is clearer when you replace the JSX with the resulting JS code:

return isTodays && React.createElement(Event.Event, { dayHeight, event: e })

See https://reactjs.org/docs/react-api.html#createelement. You never want to call the function components directly, how react works is that you always hand a reference the component to react and let it call the function at the correct time.