javascript - Multiple calls to state updater from useState in component causes multiple re-renders - Stack Overflow

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I'm trying React hooks for the first time and all seemed good until I realised that when I get data and update two different state variables (data and loading flag), my component (a data table) is rendered twice, even though both calls to the state updater are happening in the same function. Here is my api function which is returning both variables to my component.

const getData = url => {

    const [data, setData] = useState(null);
    const [loading, setLoading] = useState(true);

    useEffect(async () => {

        const test = await api.get('/people')

        if(test.ok){
            setLoading(false);
            setData(test.data.results);
        }

    }, []);

    return { data, loading };
};

In a normal class component you'd make a single call to update the state which can be a complex object but the "hooks way" seems to be to split the state into smaller units, a side effect of which seems to be multiple re-renders when they are updated separately. Any ideas how to mitigate this?

I'm trying React hooks for the first time and all seemed good until I realised that when I get data and update two different state variables (data and loading flag), my component (a data table) is rendered twice, even though both calls to the state updater are happening in the same function. Here is my api function which is returning both variables to my component.

const getData = url => {

    const [data, setData] = useState(null);
    const [loading, setLoading] = useState(true);

    useEffect(async () => {

        const test = await api.get('/people')

        if(test.ok){
            setLoading(false);
            setData(test.data.results);
        }

    }, []);

    return { data, loading };
};

In a normal class component you'd make a single call to update the state which can be a complex object but the "hooks way" seems to be to split the state into smaller units, a side effect of which seems to be multiple re-renders when they are updated separately. Any ideas how to mitigate this?

• javascript
• reactjs
• react-hooks

Share Improve this question Follow edited Dec 1, 2018 at 21:04 Yangshun Tay 53.1k3333 gold badges123123 silver badges149149 bronze badges asked Dec 1, 2018 at 20:06 jonhobbsjonhobbs 27.9k3737 gold badges117117 silver badges177177 bronze badges 5

• 4 If you have depended states you should probably use useReducer – thinklinux Commented Nov 18, 2019 at 11:29
• 1 Wow! I only I just discovered this and it has completely blown my understanding about how react rendering works. I can't understand any advantage for working this way - it seems rather arbitrary that the behaviour in an async callback is different from in a normal event handler. BTW, in my tests it seems that the reconciliation (i.e. update of the real DOM) doesn't happen until after all the setState calls have been processed, so the intermediate render calls are wasted anyway. – Andy Commented Jul 3, 2020 at 9:33
• 2 "it seems rather arbitrary that the behaviour in an async callback is different from in a normal event handler" - It is not arbitrary but rather by implementation [1]. React batches all setState invocations done during a React event handler, and applies them just before exiting its own browser event handler. However, several setStates outside of event handlers (e.g. in network responses) will not be batched. So you would get two re-renders in that case. [1] github.com/facebook/react/issues/10231#issuecomment-316644950 – soumyadityac Commented Feb 12, 2021 at 17:47
• 'but the "hooks way" seems to be to split the state into smaller units' -- this is a bit misleading, because the multiple re-renders only happen when the setX functions are called within an async callback. Sources: github.com/facebook/react/issues/14259#issuecomment-439632622, blog.logrocket.com/… – allieferr Commented Nov 11, 2021 at 2:08
• you can use useReducer or redux-toolkit – Aman Ghanghoriya Commented Oct 22, 2022 at 12:47

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9 Answers 9

Sorted by: Reset to default 213

You could combine the loading state and data state into one state object and then you could do one setState call and there will only be one render.

Note: Unlike the setState in class components, the setState returned from useState doesn't merge objects with existing state, it replaces the object entirely. If you want to do a merge, you would need to read the previous state and merge it with the new values yourself. Refer to the docs.

I wouldn't worry too much about calling renders excessively until you have determined you have a performance problem. Rendering (in the React context) and committing the virtual DOM updates to the real DOM are different matters. The rendering here is referring to generating virtual DOMs, and not about updating the browser DOM. React may batch the setState calls and update the browser DOM with the final new state.

const {useState, useEffect} = React;

function App() {
  const [userRequest, setUserRequest] = useState({
    loading: false,
    user: null,
  });

  useEffect(() => {
    // Note that this replaces the entire object and deletes user key!
    setUserRequest({ loading: true });
    fetch('https://randomuser.me/api/')
      .then(results => results.json())
      .then(data => {
        setUserRequest({
          loading: false,
          user: data.results[0],
        });
      });
  }, []);

  const { loading, user } = userRequest;

  return (
    <div>
      {loading && 'Loading...'}
      {user && user.name.first}
    </div>
  );
}

ReactDOM.render(<App />, document.querySelector('#app'));

<script src="https://unpkg.com/[email protected]/umd/react.development.js"></script>
<script src="https://unpkg.com/[email protected]/umd/react-dom.development.js"></script>
<div id="app"></div>

Alternative - write your own state merger hook

const {useState, useEffect} = React;

function useMergeState(initialState) {
  const [state, setState] = useState(initialState);
  const setMergedState = newState => 
    setState(prevState => Object.assign({}, prevState, newState)
  );
  return [state, setMergedState];
}

function App() {
  const [userRequest, setUserRequest] = useMergeState({
    loading: false,
    user: null,
  });

  useEffect(() => {
    setUserRequest({ loading: true });
    fetch('https://randomuser.me/api/')
      .then(results => results.json())
      .then(data => {
        setUserRequest({
          loading: false,
          user: data.results[0],
        });
      });
  }, []);

  const { loading, user } = userRequest;

  return (
    <div>
      {loading && 'Loading...'}
      {user && user.name.first}
    </div>
  );
}

ReactDOM.render(<App />, document.querySelector('#app'));

<script src="https://unpkg.com/[email protected]/umd/react.development.js"></script>
<script src="https://unpkg.com/[email protected]/umd/react-dom.development.js"></script>
<div id="app"></div>

This also has another solution using useReducer! first we define our new setState.

const [state, setState] = useReducer(
  (state, newState) => ({...state, ...newState}),
  {loading: true, data: null, something: ''}
)

after that we can simply use it like the good old classes this.setState, only without the this!

setState({loading: false, data: test.data.results})

As you may noticed in our new setState (just like as what we previously had with this.setState), we don't need to update all the states together! for example I can change one of our states like this (and it doesn't alter other states!):

setState({loading: false})

Awesome, Ha?!

So let's put all the pieces together:

import {useReducer} from 'react'

const getData = url => {
  const [state, setState] = useReducer(
    (state, newState) => ({...state, ...newState}),
    {loading: true, data: null}
  )

  useEffect(async () => {
    const test = await api.get('/people')
    if(test.ok){
      setState({loading: false, data: test.data.results})
    }
  }, [])

  return state
}

---

Typescript Support. Thanks to P. Galbraith who replied this solution, Those using typescript can use this:

useReducer<Reducer<MyState, Partial<MyState>>>(...)

where MyState is the type of your state object.

e.g. In our case it'll be like this:

interface MyState {
   loading: boolean;
   data: any;
   something: string;
}

const [state, setState] = useReducer<Reducer<MyState, Partial<MyState>>>(
  (state, newState) => ({...state, ...newState}),
  {loading: true, data: null, something: ''}
)

---

Previous State Support. In comments user2420374 asked for a way to have access to the prevState inside our setState, so here's a way to achieve this goal:

const [state, setState] = useReducer(
    (state, newState) => {
        newWithPrevState = isFunction(newState) ? newState(state) : newState
        return (
            {...state, ...newWithPrevState}
        )
     },
     initialState
)

// And then use it like this...
setState(prevState => {...})

isFunction checks whether the passed argument is a function (which means you're trying to access the prevState) or a plain object. You can find this implementation of isFunction by Alex Grande here.

---

Notice. For those who want to use this answer a lot, I decided to turn it into a library. You can find it here:

Github: https://github.com/thevahidal/react-use-setstate

NPM: https://www.npmjs.com/package/react-use-setstate

Batching update in react-hooks https://github.com/facebook/react/issues/14259

React currently will batch state updates if they're triggered from within a React-based event, like a button click or input change. It will not batch updates if they're triggered outside of a React event handler, like an async call.

This will do:

const [state, setState] = useState({ username: '', password: ''});

// later
setState({
    ...state,
    username: 'John'
});

To replicate this.setState merge behavior from class components, React docs recommend to use the functional form of useState with object spread - no need for useReducer:

setState(prevState => {
  return {...prevState, loading, data};
});

The two states are now consolidated into one, which will save you a render cycle.

There is another advantage with one state object: loading and data are dependent states. Invalid state changes get more apparent, when state is put together:

setState({ loading: true, data }); // ups... loading, but we already set data

You can even better ensure consistent states by 1.) making the status - loading, success, error, etc. - explicit in your state and 2.) using useReducer to encapsulate state logic in a reducer:

const useData = () => {
  const [state, dispatch] = useReducer(reducer, /*...*/);

  useEffect(() => {
    api.get('/people').then(test => {
      if (test.ok) dispatch(["success", test.data.results]);
    });
  }, []);
};

const reducer = (state, [status, payload]) => {
  if (status === "success") return { ...state, data: payload, status };
  // keep state consistent, e.g. reset data, if loading
  else if (status === "loading") return { ...state, data: undefined, status };
  return state;
};

const App = () => {
  const { data, status } = useData();
  return status === "loading" ? <div> Loading... </div> : (
    // success, display data 
  )
}

const useData = () => {
  const [state, dispatch] = useReducer(reducer, {
    data: undefined,
    status: "loading"
  });

  useEffect(() => {
    fetchData_fakeApi().then(test => {
      if (test.ok) dispatch(["success", test.data.results]);
    });
  }, []);

  return state;
};

const reducer = (state, [status, payload]) => {
  if (status === "success") return { ...state, data: payload, status };
  // e.g. make sure to reset data, when loading.
  else if (status === "loading") return { ...state, data: undefined, status };
  else return state;
};

const App = () => {
  const { data, status } = useData();
  const count = useRenderCount();
  const countStr = `Re-rendered ${count.current} times`;

  return status === "loading" ? (
    <div> Loading (3 sec)... {countStr} </div>
  ) : (
    <div>
      Finished. Data: {JSON.stringify(data)}, {countStr}
    </div>
  );
}

//
// helpers
//

const useRenderCount = () => {
  const renderCount = useRef(0);
  useEffect(() => {
    renderCount.current += 1;
  });
  return renderCount;
};

const fetchData_fakeApi = () =>
  new Promise(resolve =>
    setTimeout(() => resolve({ ok: true, data: { results: [1, 2, 3] } }), 3000)
  );

ReactDOM.render(<App />, document.getElementById("root"));

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.13.0/umd/react.production.min.js" integrity="sha256-32Gmw5rBDXyMjg/73FgpukoTZdMrxuYW7tj8adbN8z4=" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.13.0/umd/react-dom.production.min.js" integrity="sha256-bjQ42ac3EN0GqK40pC9gGi/YixvKyZ24qMP/9HiGW7w=" crossorigin="anonymous"></script>
<div id="root"></div>
<script>var { useReducer, useEffect, useState, useRef } = React</script>

PS: Make sure to prefix custom Hooks with use (useData instead of getData). Also passed callback to useEffect cannot be async.

If you are using third-party hooks and can't merge the state into one object or use useReducer, then the solution is to use :

ReactDOM.unstable_batchedUpdates(() => { ... })

Recommended by Dan Abramov here

See this example

A little addition to answer https://stackoverflow.com/a/53575023/121143

Cool! For those who are planning to use this hook, it could be written in a bit robust way to work with function as argument, such as this:

const useMergedState = initial => {
  const [state, setState] = React.useState(initial);
  const setMergedState = newState =>
    typeof newState == "function"
      ? setState(prevState => ({ ...prevState, ...newState(prevState) }))
      : setState(prevState => ({ ...prevState, ...newState }));
  return [state, setMergedState];
};

Update: optimized version, state won't be modified when incoming partial state was not changed.

const shallowPartialCompare = (obj, partialObj) =>
  Object.keys(partialObj).every(
    key =>
      obj.hasOwnProperty(key) &&
      obj[key] === partialObj[key]
  );

const useMergedState = initial => {
  const [state, setState] = React.useState(initial);
  const setMergedState = newIncomingState =>
    setState(prevState => {
      const newState =
        typeof newIncomingState == "function"
          ? newIncomingState(prevState)
          : newIncomingState;
      return shallowPartialCompare(prevState, newState)
        ? prevState
        : { ...prevState, ...newState };
    });
  return [state, setMergedState];
};

In addition to Yangshun Tay's answer you'll better to memoize setMergedState function, so it will return the same reference each render instead of new function. This can be crucial if TypeScript linter forces you to pass setMergedState as a dependency in useCallback or useEffect in parent component.

import {useCallback, useState} from "react";

export const useMergeState = <T>(initialState: T): [T, (newState: Partial<T>) => void] => {
    const [state, setState] = useState(initialState);
    const setMergedState = useCallback((newState: Partial<T>) =>
        setState(prevState => ({
            ...prevState,
            ...newState
        })), [setState]);
    return [state, setMergedState];
};

You can also use useEffect to detect a state change, and update other state values accordingly

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