Error while trying to use ref inside useMemo() - reactjs

Question

I want to scroll FlatList to the certain index when screen is launched and data for the list is retrieved from the server.
I have a problem of using ref inside useMemo(). I'm getting an error:
Cannot read property current of undefined.
How to fix this error? Does my approach correct?
Here is what I'm doing:
const WeeklyMeetings = props => {
const [meetings, setMeetings] = useState(null)
useEffect(() => {
AllMeeting() // getting data from the server
}, [])
const getResult = useMemo(() => {
flatListRef.current.scrollToIndex({index: 15, animated: true })
}, [meetings]);
const flatListRef = useRef();
const AllMeeting = async (id) => {
setLoading(true)
try {
const meetings = await meeting.allMeetingsAsc(id)
setMeetings(meetings)
} catch (error) {
console.log("error ", error)
}
}
return (
<View style={styles.rootContainer}>
<FlatList
ref={flatListRef}
style={styles.list}
data={meetings}
renderItem={renderItem}
onScrollToIndexFailed={()=>{}}
/>
</View>
)
}

Answer 1

The ref needs to be defined before using it.
Also since you want to just scroll to index when you receive meeting value, you can make use of useEffect hook.
Also note that you only want to scrollToIndex once value meetings is available and hence you can skip the initial call to useEffect by keeping track of initialRender
const WeeklyMeetings = props => {
const [meetings, setMeetings] = useState(null)
useEffect(() => {
const AllMeeting = async (id) => {
setLoading(true)
try {
const meetings = await meeting.allMeetingsAsc(id)
setMeetings(meetings)
} catch (error) {
console.log("error ", error)
}
}
AllMeeting();
}, [])
const flatListRef = useRef();
const initialRender = useRef(true)
useEffect(() => {
if(!initialRender.current) {
flatListRef.current.scrollToIndex({index: 15, animated: true })
} else {
initialRender.current = false;
}
}, [meetings])
return (
<View style={styles.rootContainer}>
<FlatList
ref={flatListRef}
style={styles.list}
data={meetings}
renderItem={renderItem}
getItemLayout={(data, index) => (
{length: 50, offset: 50 * index, index}
)}
/>
</View>
)
}
According to Documentation
You would need to implement a getItemLayout function for FlatList
since scrollToIndex Cannot scroll to locations outside the render
window without specifying the getItemLayout prop.