We can use the new useEffect() hook to simulate componentDidUpdate(), but it seems like useEffect() is being ran after every render, even the first time. How do I get it to not run on initial render? If we want the effect to run in the same phase that componentDidUpdate does, we can use useLayoutEffect instead. We can use the useRef hook to store any mutable value we like, so we could use that to keep track of if it's the first time the useEffect function is being run. componentDidUpdate() is invoked immediately after updating occurs. This method is not called for the initial render.
const didMountRef = useRef(false);
useEffect(() => {
// Are we on he initial render? No! Let's run some code:
if (didMountRef.current) {
// Run some code...
} else {
didMountRef.current = true;
}
}, [dependency]);const { useState, useRef, useLayoutEffect } = React;
function ComponentDidUpdateFunction() {
const [count, setCount] = useState(0);
const firstUpdate = useRef(true);
useLayoutEffect(() => {
if (firstUpdate.current) {
firstUpdate.current = false;
return;
}
console.log("componentDidUpdateFunction");
});
return (
<div>
<p>componentDidUpdateFunction: {count} times</p>
<button
onClick={() => {
setCount(count + 1);
}}
>
Click Me
</button>
</div>
);
}
ReactDOM.render(
<ComponentDidUpdateFunction />,
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>Usage example:
import React, { useState, useEffect } from 'react';
import useDidMountEffect from '../path/to/useDidMountEffect';
const MyComponent = (props) => {
const [state, setState] = useState({
key: false
});
useEffect(() => {
// you know what is this, don't you?
}, []);
useDidMountEffect(() => {
// react please run me if 'key' changes, but not on initial render
}, [state.key]);
return (
<div>
...
</div>
);
}
// ...
Changing state will always cause a re-render. By default, useEffect always runs after render has run. This means if you don’t include a dependency array when using useEffect to fetch data, and use useState to display it, you will always trigger another render after useEffect runs. Important: the useEffect hook will always run on mount regardless of if there is anything in its dependency array.28-Jun-2021 Note: Although useEffect is deferred until after the browser has painted, it’s guaranteed to fire before any new renders. React will always flush a previous render’s effects before starting a new update.08-Dec-2021
In this tutorial, we will try to find the solution to React Useeffect Avoid Initial Render through programming. The following code illustrates this.
const didMountRef = useRef(false);
useEffect(() => {
// Are we on he initial render? No! Let's run some code:
if (didMountRef.current) {
// Run some code...
} else {
didMountRef.current = true;
}
}, [dependency]);Sometimes we don’t want a useEffect hook to run on initial render. This could be for many reasons, but a common use case is when we’re fetching the data on which that hook depends. Initially, the data is empty, so running that hook is pointless. On initial render, we fetch the data, but we also run our second effect. In this case, we’ll create a ref to a boolean that tracks whether the component has mounted. It will start out as false, but once the effect runs for the first time, we can change it to true.
function MyComponent() {
const [data, setData] = useState();
// An effect to fetch the data
useEffect(() => {
fetch('/api/some-api')
.then((res) => res.json())
.then((d) => {
setData(d);
});
}, []);
// Do something else with the data
useEffect(() => {
doSomething(data);
}, [data]);
}function MyComponent() {
const [data, setData] = useState();
const isMounted = useRef(false);
// An effect to fetch the data
useEffect(() => {
fetch('/api/some-api')
.then((res) => res.json())
.then((d) => {
setData(d);
});
}, []);
// Do something else with the data
useEffect(() => {
if (isMounted.current) {
doSomething(data);
} else {
isMounted.current = true;
}
}, [data]);
}Does useEffect run after every render? Yes! By default, it runs both after the first render and after every update. (We will later talk about how to customize this.) Instead of thinking in terms of “mounting” and “updating”, you might find it easier to think that effects happen “after render”. React guarantees the DOM has been updated by the time it runs the effects. If you’re used to classes, you might be wondering why the effect cleanup phase happens after every re-render, and not just once during unmounting. Let’s look at a practical example to see why this design helps us create components with fewer bugs. We’ve learned that useEffect lets us express different kinds of side effects after a component renders. Some effects might require cleanup so they return a function:
import React, { useState, useEffect } from 'react';
function Example() {
const [count, setCount] = useState(0);
// Similar to componentDidMount and componentDidUpdate: useEffect(() => { // Update the document title using the browser API document.title = `You clicked ${count} times`; });
return (
<div>
<p>You clicked {count} times</p>
<button onClick={() => setCount(count + 1)}>
Click me
</button>
</div>
);
}class Example extends React.Component {
constructor(props) {
super(props);
this.state = {
count: 0
};
}
componentDidMount() { document.title = `You clicked ${this.state.count} times`; } componentDidUpdate() { document.title = `You clicked ${this.state.count} times`; }
render() {
return (
<div>
<p>You clicked {this.state.count} times</p>
<button onClick={() => this.setState({ count: this.state.count + 1 })}>
Click me
</button>
</div>
);
}
}import React, { useState, useEffect } from 'react';
function Example() {
const [count, setCount] = useState(0);
useEffect(() => { document.title = `You clicked ${count} times`; });
return (
<div>
<p>You clicked {count} times</p>
<button onClick={() => setCount(count + 1)}>
Click me
</button>
</div>
);
}class FriendStatus extends React.Component {
constructor(props) {
super(props);
this.state = {
isOnline: null
};
this.handleStatusChange = this.handleStatusChange.bind(this);
}
componentDidMount() {
ChatAPI.subscribeToFriendStatus(this.props.friend.id, this.handleStatusChange);
}
componentWillUnmount() {
ChatAPI.unsubscribeFromFriendStatus(this.props.friend.id, this.handleStatusChange);
}
handleStatusChange(status) {
this.setState({
isOnline: status.isOnline
});
}
render() {
if (this.state.isOnline === null) {
return 'Loading...';
}
return this.state.isOnline ? 'Online' : 'Offline';
}
}import React, {
useState,
useEffect
} from 'react';
function FriendStatus(props) {
const [isOnline, setIsOnline] = useState(null);
useEffect(() => {
function handleStatusChange(status) {
setIsOnline(status.isOnline);
}
ChatAPI.subscribeToFriendStatus(props.friend.id, handleStatusChange); // Specify how to clean up after this effect: return function cleanup() { ChatAPI.unsubscribeFromFriendStatus(props.friend.id, handleStatusChange); }; });
if (isOnline === null) {
return 'Loading...';
}
return isOnline ? 'Online' : 'Offline';
}You may want to run the side effect just once after the initial render. A typical case will be fetching data making an API call, and storing the response in the state variable after the initial render. You do not want to make this API call again. In this case, the side effect runs only once after the initial render of the component. The cleanup function gets invoked every time after the initial render to clean up the previous side effect, and then the subsequent side effect runs.
So, a side effect is something that we should isolate from the rendering. The useEffect hook helps perform side effects by isolating it from the rendering logic.
useEffect(callback, [dependencies]);The first is the default case. If you do not pass the dependency array to the useEffect hook, the callback function executes on every render. Thus React will run the side effect defined in it after every render.
useEffect(() => {
// Side Effect
});You can pass an empty array as the second argument to the useEffect hook to tackle this use case.
useEffect(() => {
// Side Effect
}, []);In this case, the side effect will run every time the value of the state variable changes. If multiple state variables influence a side effect, you can pass them as comma-separated in the dependency array.
useEffect(() => {
// Side Effect
}, [state1, state2, state3]);Just like the state, we can also use props as a dependency to run the side effect. In this case, the side effect will run every time there is a change to the props passed as a dependency.
useEffect(() => {
// Side Effect
}, [props]);We create the useDidMountEffect hook to track whether the first render is done. Then we can check the ref’s value to see when the first render is done and run the function we want when the first render is done. We can create a hook to check whether the first render is done before running the code we want. In this article, we’ll look at how to make the React useEffect callback not run on initial render.
To do this, we write:
import React, { useEffect, useRef, useState } from "react";
const useDidMountEffect = (func, deps) => {
const didMount = useRef(false);
useEffect(() => {
if (didMount.current) {
func();
} else {
didMount.current = true;
}
}, deps);
};
export default function App() {
const [count, setCount] = useState(0);
useDidMountEffect(() => {
console.log("second render");
});
return (
<div className="App">
<button onClick={() => setCount((c) => c + 1)}>increment</button>
<p>{count}</p>
</div>
);
}
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. Paul Razvan Berg Permalink to comment# November 25, 2019 The issue with this approach is that it will fire a warning if you depend on any props or state (due to the react-hooks/exhaustive-deps eslint rule). Better define a custom Hook in your codebase: const useEffectOnlyOnce = (func) => useEffect(func, []) And import it and use it. If you use this optimization, make sure the array includes all values from the component scope (such as props and state) that change over time and that are used by the effect. Otherwise, your code will reference stale values from previous renders.
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.
import React, { useEffect } from 'react';
function App() {
useEffect(() => {
// Run! Like go get some data from an API.
});
return (
<div>
{/* Do something with data. */}
</div>
);
}The second param is an array of variables that the component will check to make sure changed before re-rendering. You could put whatever bits of props and state you want in here to check against.
Or, put nothing:
import React, { useEffect } from 'react';
function App() {
useEffect(() => {
// Run! Like go get some data from an API.
}, []);
return (
<div>
{/* Do something with data. */}
</div>
);
}The issue with this approach is that it will fire a warning if you depend on any props or state (due to the react-hooks/exhaustive-deps eslint rule).
Better define a custom Hook in your codebase:
const useEffectOnlyOnce = (func) => useEffect(func, [])name prop is mentioned in the dependencies argument of useEffect(..., [name]). useEffect() hook runs the side-effect after initial rendering, and on later renderings only if the name value changes. Using the dependencies argument of useEffect() you control when to invoke the side-effect, independently from the rendering cycles of the component. Again, that's the essence of useEffect() hook. useEffect(..., []) was supplied with an empty array as the dependencies argument. When configured in such a way, the useEffect() executes the callback just once, after initial mounting.
The component rendering and side-effect logic are independent. It would be a mistake to perform side-effects directly in the body of the component, which is primarily used to compute the output.
How often the component renders isn't something you can control — if React wants to render the component, you cannot stop it.
jsxfunction Greet({ name }) { const message = `Hello, ${name}!`; // Calculates output // Bad! document.title = `Greetings to ${name}`; // Side-effect! return <div>{message}</div>; // Calculates output}How to decouple rendering from the side-effect? Welcome useEffect() — the hook that runs side-effects independently of rendering.
jsximport { useEffect } from 'react';function Greet({ name }) { const message = `Hello, ${name}!`; // Calculates output useEffect(() => { // Good! document.title = `Greetings to ${name}`; // Side-effect! }, [name]); return <div>{message}</div>; // Calculates output}useEffect() hook accepts 2 arguments:
javascriptuseEffect(callback[, dependencies]);dependencies argument of useEffect(callback, dependencies) lets you control when the side-effect runs. When dependencies are:
A) Not provided: the side-effect runs after every rendering.
jsximport {
useEffect
}
from 'react';
function MyComponent() {
useEffect(() => { // Runs after EVERY rendering }); }B) An empty array []: the side-effect runs once after the initial rendering.
jsximport {
useEffect
}
from 'react';
function MyComponent() {
useEffect(() => { // Runs ONCE after initial rendering }, []);}