How to call loading function with React useEffect only once

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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 (...);
}

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>;
}

Run useEffect Only Once, React has a built-in hook called useEffect. Hooks are used in function components. The Class component comparison to useEffect are the  React has a built-in hook called useEffect. Hooks are used in function components. The Class component comparison to useEffect are the methods componentDidMount, componentDidUpdate, and componentWillUnmount. useEffect will run when the component renders, which might be more times than you think. I feel like I’ve had this come up a dozen times in the past few weeks, so it seems worthy of a quick blog post.


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.

Run useEffect Only Once, import React, { useEffect } from 'react'; function App() { useEffect(() => { // Run! Like go get some In a totally isolated example like that, it's likely the useEffect will run only once. But in a NextThe Simplest Way to Load CSS Asynchronously ». The following functional React component runs the useEffect hook twice, even though an empty array [] is passed as a second argument and the component is added to the page only once: const Tips = () => { const [idx, setIdx] = useState(0) useEffect( () => { console.log('start use effect') const interval = setInterval( () => { setIdx(idx + 1) }, 1000) }, []) // empty array: run effect only once return <> {idx}</> }


useMountEffect hook

Running a function only once after component mounts is such a common pattern that it justifies a hook of it's 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.

Using the Effect Hook – React, import React, { useState, useEffect } from 'react'; function Example() { const [count, setCount] = useState(0); // Similar to isOnline === null) { return 'Loading. However, as we learned earlier, effects run for every render and not just once. Put the console.log inside the useEffect. Probably you have other side effects that cause the component to rerender but the useEffect itself will only be called once.


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/react@16.7.0-alpha.0/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom@16.7.0-alpha.0/umd/react-dom.development.js"></script>

<div id="app"></div>

React useEffect hook with code examples, React useEffect is a hook that gets triggered for componentDidMount, now is to execute the setMessage function only on the componentDidMount lifecycle. If you take a look at the console log it only shows “trigger use effect hook” once. I would like to call an async function and get the result for my UseEffect. The fetch api examples i found on the internet are directly made in the useEffect function. If my URL changes, i must patch all my fetchs.


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, [])

How to call loading function with React useEffect only once, 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  When the user clicks the submit button, BasicForm should 'touch' all inputs and only then call validateAllFields(), because validation errors will only show if an input has been touched. So if the user hasn't touched any, BasicForm needs to make sure to 'touch' every input before calling the validateAllFields() function.


Why is my useEffect Hook executed in an endless loop? » Andreas , So there it is – our Hook that is going to load some Users from an API to display it and – if available – the cleanup function we just wrote are executed endlessly? If you do, each time one of those objects change, React will execute In our example case we only want to execute useEffect once when it is  what is the difference between first and last useEffects, first useEffect will be invoked on the willmount or didmount, last useEffect was returned call back function with empty array why? could you elaborate each useEffect use cases when and how we can use? – siluveru kiran kumar Aug 2 '19 at 10:46


useEffect(fn, []) is not the new componentDidMount(), "What is the hooks equivalent to [some lifecycle method]? Ah, I see that useEffect with an empty dependency array does just that. Thinking in terms of time, like "call my side effect once on mount" can hinder your learning of hooks. When the page loads, you have three seconds to click the button a few  Can I call a function outside the useEffect? I know the answer should probably be "no" or something in the lines of "useCallback should be used", but let's say we have a computation that is not dependent on anything, and it starts working as soon as component mounts.


Create a componentDidMount useEffect hook in React, In this lesson we'll explore how to create `useEffect` call, and listen to the window This function will run every single time our component re-renders. In order to register just once we need to pass in an empty array as the second argument  React uses the 2nd argument to determine whether or not it needs to execute the function passed to useEffect by passing nothing, React will run the effect every time. This may cause performance issues or just be a tad overkill, so let's see how to add a little extra control to when our effect functions are run. Run the effect only once