How to call loading function with React useEffect only once

Question

The useEffect React hook will run the passed-in function on every change. This can be optimized to let it call only when the desired properties change.

What if I want to call an initialization function from componentDidMount and not call it again on changes? Let's say I want to load an entity, but the loading function doesn't need any data from the component. How can we make this using the useEffect hook?

class MyComponent extends React.PureComponent {
    componentDidMount() {
        loadDataOnlyOnce();
    }
    render() { ... }
}

With hooks this could look like this:

function MyComponent() {
    useEffect(() => {
        loadDataOnlyOnce(); // this will fire on every change :(
    }, [...???]);
    return (...);
}

Answer 1

If you only want to run the function given to useEffect after the initial render, you can give it an empty array as second argument.

function MyComponent() {
  useEffect(() => {
    loadDataOnlyOnce();
  }, []);

  return <div> {/* ... */} </div>;
}

Answer 2

TL;DR

useEffect(yourCallback, []) - will trigger the callback only after the first render.

Detailed explanation

useEffect runs by default after every render of the component (thus causing an effect).

When placing useEffect in your component you tell React you want to run the callback as an effect. React will run the effect after rendering and after performing the DOM updates.

If you pass only a callback - the callback will run after each render.

If passing a second argument (array), React will run the callback after the first render and every time one of the elements in the array is changed. for example when placing useEffect(() => console.log('hello'), [someVar, someOtherVar]) - the callback will run after the first render and after any render that one of someVar or someOtherVar are changed.

By passing the second argument an empty array, React will compare after each render the array and will see nothing was changed, thus calling the callback only after the first render.

Answer 3

useMountEffect hook

Running a function only once after component mounts is such a common pattern that it justifies a hook of its own that hides implementation details.

const useMountEffect = (fun) => useEffect(fun, [])

Use it in any functional component.

function MyComponent() {
    useMountEffect(function) // function will run only once after it has mounted. 
    return <div>...</div>;
}

---

About the useMountEffect hook

When using useEffect with a second array argument, React will run the callback after mounting (initial render) and after values in the array have changed. Since we pass an empty array, it will run only after mounting.

Answer 4

We have to stop thinking in component-life-cycle-methods (i.e. componentDidMount). We have to start thinking in effects. React effects are different from old-style class-life-cycle-methods.

By default effects run after every render cycle, but there are options to opt out from this behaviour. To opt out, you can define dependencies that mean that an effect is only carried out when a change to one of the dependencies is made.

If you explicitly define, that an effect has no dependecy, the effect runs only once, after the first render-cycle.

1st solution (with ESLint-complaint)

So, the first solution for your example would be the following:

function MyComponent() {

    const loadDataOnlyOnce = () => {
      console.log("loadDataOnlyOnce");
    };

    useEffect(() => {
        loadDataOnlyOnce(); // this will fire only on first render
    }, []);
    return (...);
}

But then the React Hooks ESLint plugin will complain with something like that:

React Hook useEffect has missing dependency: loadDataOnlyOnce. Either include it or remove the dependency array.

At first this warning seems annoying, but please don't ignore it. It helps you code better and saves you from "stale closures". If you don't know what "stale closures" are, please read this great post.

2nd solution (the right way, if dependency is not dependent on component)

If we add loadDataOnlyOnce to the dependency array, our effect will run after every render-cycle, because the reference of loadDataOnlyOnce changes on every render, because the function is destroyed(garbarge-collected) and a new function is created, but that's exactly what we don't want.

We have to keep the same reference of loadDataOnlyOnce during render-cycles.

So just move the function-definition above:

const loadDataOnlyOnce = () => {
  console.log("loadDataOnlyOnce");
};

function MyComponent() {
    useEffect(() => {
        loadDataOnlyOnce(); // this will fire only on first render
    }, [loadDataOnlyOnce]);
    return (...);
}

With this change you ensure that the reference of loadDataOnlyOnce will never change. Therefore you can also safely add the reference to the dependency array.

3rd solution (the right way, if dependency is dependent on component)

If the dependency of the effect (loadDataOnlyOnce), is dependent on the component (need props or state), there's React's builtin useCallback-Hook.

An elementary sense of the useCallback-Hook is to keep the reference of a function identical during render-cycles.

function MyComponent() {
    const [state, setState] = useState("state");

    const loadDataOnlyOnce = useCallback(() => {
      console.log(`I need ${state}!!`);
    }, [state]);

    useEffect(() => {
        loadDataOnlyOnce(); // this will fire only when loadDataOnlyOnce-reference changes
    }, [loadDataOnlyOnce]);
    return (...);
}

Answer 5

function useOnceCall(cb, condition = true) {
  const isCalledRef = React.useRef(false);

  React.useEffect(() => {
    if (condition && !isCalledRef.current) {
      isCalledRef.current = true;
      cb();
    }
  }, [cb, condition]);
}

and use it.

useOnceCall(() => {
  console.log('called');
})

or

useOnceCall(()=>{
  console.log('Fetched Data');
}, isFetched);

Answer 6

Pass an empty array as the second argument to useEffect. This effectively tells React, quoting the docs:

This tells React that your effect doesn’t depend on any values from props or state, so it never needs to re-run.

Here's a snippet which you can run to show that it works:

function App() {
  const [user, setUser] = React.useState(null);

  React.useEffect(() => {
    fetch('https://randomuser.me/api/')
      .then(results => results.json())
      .then(data => {
        setUser(data.results[0]);
      });
  }, []); // Pass empty array to only run once on mount.
  
  return <div>
    {user ? user.name.first : 'Loading...'}
  </div>;
}

ReactDOM.render(<App/>, document.getElementById('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>

Answer 7

I had this issue with React 18. This is how I handled it:

import { useEffect, useRef } from "react";

export default function Component() {
    const wasCalled = useRef(false);

    useEffect(() => {
        if(wasCalled.current) return;
        wasCalled.current = true;
            
         /* CODE THAT SHOULD RUN ONCE */

    }, []);

    return <div> content </div>;
}

Check here how they explain why useEffect is called more than once.

Answer 8

I like to define a mount function, it tricks EsLint in the same way useMount does and I find it more self-explanatory.

const mount = () => {
  console.log('mounted')
  // ...

  const unmount = () => {
    console.log('unmounted')
    // ...
  }
  return unmount
}
useEffect(mount, [])

Answer 9

leave the dependency array blank . hope this will help you understand better.

   useEffect(() => {
      doSomething()
    }, []) 

empty dependency array runs Only Once, on Mount

useEffect(() => {
  doSomething(value)
}, [value])  

pass value as a dependency. if dependencies has changed since the last time, the effect will run again.

useEffect(() => {
  doSomething(value)
})  

no dependency. This gets called after every render.

Answer 10

Here is my version of Yasin's answer.

import {useEffect, useRef} from 'react';

const useOnceEffect = (effect: () => void) => {
  const initialRef = useRef(true);

  useEffect(() => {
    if (!initialRef.current) {
      return;
    }
    initialRef.current = false;
    effect();
  }, [effect]);
};

export default useOnceEffect;

Usage:

useOnceEffect(
  useCallback(() => {
    nonHookFunc(deps1, deps2);
  }, [deps1, deps2])
);

Answer 11

You should do the first way below

useEffect hook has 3ways to use.

1. runs once when the component will be rendered:

    useEffect(()=>{
        // your code
    },[])
   

2. runs everytime when these x1,x2,... dependencies have been chaged:

    useEffect(()=>{
    // your code
    },[x1,x2,...])
   

3. runs after updating anything and also on rendering the component:

     useEffect(()=>{
    // your code
     })
   

Answer 12

This does not answer your question exactly but may have the same intended affect of only running a function once and after the first render. Very similar to the componentDidMount function. This uses useState instead of useEffect to avoid dependency lint errors. You simply pass a self-executing anonymous function as the first argument to useState. As an aside, I'm not sure why React doesn't simply provide a hook that does this.

import React, { useState } from "react"

const Component = () => {

  useState((() => {
    console.log('componentDidMountHook...')
  }))

  return (
    <div className='component'>Component</div>
  )
}

export default Component

Answer 13

I found out after some time spend on the internet. useEffect fires once on component mount, then componennt unmounts and mounts again, useEffect fires again. You have to check more on React docs, why they do that.

So, I used custom hook for that. On unmount you have to change your useRef state. In this case do not forget a return statement: when component unmounts, useEffect runs cleanup function after return.

import React, { useEffect, useRef } from "react"

const useDidMountEffect = (
  func: () => void,
  deps: React.DependencyList | undefined
) => {
  const didMount = useRef(false)

  useEffect(() => {
    if (didMount.current) {
      func()
    }
    return () => {
      didMount.current = true
    }
    // eslint-disable-next-line react-hooks/exhaustive-deps
  }, deps)
}

export default useDidMountEffect

Use it like normal useEffect:

  useDidMountEffect(() => {
    // your action    
  }, [])

Answer 14

In the future we will have useEffectEvent, see this official react article:

Use a special Hook called useEffectEvent to extract this non-reactive logic out of your Effect:

function ChatRoom({ roomId, theme }) {
  const onConnected = useEffectEvent(() => {
    showNotification('Connected!', theme);
  });

  useEffect(() => {
    const connection = createConnection(serverUrl, roomId);
    connection.on('connected', () => {
      onConnected();
    });
    connection.connect();
    return () => connection.disconnect();
  }, [roomId]); // ✅ All dependencies declared
  // ...

Here, onConnected is called an Effect Event. It’s a part of your Effect logic, but it behaves a lot more like an event handler. The logic inside it is not reactive, and it always “sees” the latest values of your props and state.

In their example even if theme changed (or the callback in the case of the question) the effect event is not run again (i.e. useEffectEvent doesn't have any dependencies).

So to answer the question (assuming loadDataOnlyOnce depends on some props/state or other variable in scope which would execute it again and again; otherwise just put it outside of the component body; react advices against useCallback for flow control):

function MyComponent() {
    const loadDataOnlyOnceEvent = useEffectEvent(loadDataOnlyOnce)

    useEffect(() => {
        loadDataOnlyOnceEvent();
    }, []);
    return (...);
}

Answer 15

window.onpageshow works even if the user presses the back button to navigate to the page, unlike passing an empty array as second argument of the use-effect hook which does not fire when returning to the page via the back button (thus not on every form of initial page load).

 useEffect(() => {    
     window.onpageshow = async function() {
      setSomeState(false)
      let results = await AsyncFunction() 
       console.log(results, 'Fires on on first load, 
        refresh, or coming to the page via the back button')
    };
 };

Answer 16

I found that with the once function from lodash the problem may be solved concisely and elegantly.

import { once } from "lodash";
import { useEffect, useRef } from "react";

export const useEffectOnce = (cb: () => void) => {
  const callBackOnce = useRef(once(cb)).current;
  useEffect(() => callBackOnce(), [callBackOnce]);
};

Answer 17

Incase you just call the function in useeffect after render you add an empty array as the second argument for the useeffect

useEffect=(()=>{
   functionName(firstName,lastName);
},[firstName,lastName])