react hook useffect how to trigger only when json data change?

Question

I am working on React hook and when i set dependency on useEffect it trigger all time,but it should trigger only when there will be some changes in Json API, Here is my code so far i did.How to make it that its trigger only when new changes will be in API.

Thanks

const [data, setData] = useState([]);
  useEffect(() => {
    const getdata = () => {
      fetch(config.auth.notification_list, config.head)
        .then((response) => response.json())
        .then((data) => {
          setData(data);
        });
    };

    getdata();
  }, [data]);

Looks like your code example doesn`t make a lot of sense. This can case an infinite loop. Make a call to api each time when data changed and than change data. — Taras Kumpanenko
– Taras Kumpanenko, Commented Dec 22, 2020 at 12:49
How will your app know that there are "some changes in the API"? — Konstantin
– Konstantin, Commented Dec 22, 2020 at 13:01
are you asking how to detect change in the server? it seems the socket io or some sort of notification from the server side is required... am i right? — adir abargil
– adir abargil, Commented Dec 22, 2020 at 13:21
I am asking that if there will be new data in API it should automatically update fetch. don't need to refresh the page then get data @adirabargil — Piotr
– Piotr, Commented Dec 22, 2020 at 13:36
of course not... but how do plan to know when the data is changing? are you planning to fetch it endlessly until there is a change? — adir abargil
– adir abargil, Commented Dec 22, 2020 at 13:37

kca · Accepted Answer · 2020-12-22 16:00:15Z

As you want to "trigger only when ... changes ...", you need to set data only if these changes happen.

Here you need to implement dataHasChanged( data ) according to your needs (you didn't specify what "API change" means exactly).

const [ data, setData ] = useState([]);
useEffect(() => {
  fetch( /*...*/ )
    .then(( data ) => {

      if( dataHasChanged( data ) ){  // <----- only set data if necessary
        setData( data );
      }

    });
}, [ data ]);

Alternatively (if appropriate for your use case), you might only fetch the new data if you know it might have changed:

const [ data, setData ] = useState([]);
useEffect(() => {

  if( !newDataNeedsToBeFetched( data ) ){ // <----- skip if no fetch necessary
    return;
  }

  fetch( /* ... */ )
    .then(( data ) => {
      setData( data );
    });
}, [ data ]);

implement `hasChanged()`

How to determine if data has changed in javascript is a separate question. Maybe you even don't have a problem there, otherwise please refer to other answers about that, e.g. design-pattern-to-check-if-a-javascript-object-has-changed.

You might also want to know about using useRef() to store the previous value: how-to-compare-oldvalues-and-newvalues-on-react-hooks-useeffect

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implement `hasChanged()`

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implement `hasChanged()`