According to the docs:
componentDidUpdate()
is invoked immediately after updating occurs. This method is not called for the initial render.
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?
As you can see in the example below, componentDidUpdateFunction
is printed during the initial render but componentDidUpdateClass
was not printed during the initial render.
function ComponentDidUpdateFunction() {
const [count, setCount] = React.useState(0);
React.useEffect(() => {
console.log("componentDidUpdateFunction");
});
return (
<div>
<p>componentDidUpdateFunction: {count} times</p>
<button
onClick={() => {
setCount(count + 1);
}}
>
Click Me
</button>
</div>
);
}
class ComponentDidUpdateClass extends React.Component {
constructor(props) {
super(props);
this.state = {
count: 0,
};
}
componentDidUpdate() {
console.log("componentDidUpdateClass");
}
render() {
return (
<div>
<p>componentDidUpdateClass: {this.state.count} times</p>
<button
onClick={() => {
this.setState({ count: this.state.count + 1 });
}}
>
Click Me
</button>
</div>
);
}
}
ReactDOM.render(
<div>
<ComponentDidUpdateFunction />
<ComponentDidUpdateClass />
</div>,
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>
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.
If we want the effect to run in the same phase that componentDidUpdate
does, we can use useLayoutEffect
instead.
Example
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/[email protected]/umd/react.development.js"></script>
<script src="https://unpkg.com/[email protected]/umd/react-dom.development.js"></script>
<div id="app"></div>
You can turn it into custom hooks, like so:
import React, { useEffect, useRef } from 'react';
const useDidMountEffect = (func, deps) => {
const didMount = useRef(false);
useEffect(() => {
if (didMount.current) func();
else didMount.current = true;
}, deps);
}
export default useDidMountEffect;
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>
);
}
// ...
@MehdiDehghani, your solution work perfectly fine, one addition you have to do is on unmount, reset the didMount.current
value to false
. When to try to use this custom hook somewhere else, you don't get cache value.
import React, { useEffect, useRef } from 'react';
const useDidMountEffect = (func, deps) => {
const didMount = useRef(false);
useEffect(() => {
if (didMount.current) func();
else didMount.current = true;
return () => {
didMount.current = false;
}
}, deps);
}
export default useDidMountEffect;
@ravi, yours doesn't call the passed-in unmount function. Here's a version that's a little more complete:
/**
* Identical to React.useEffect, except that it never runs on mount. This is the equivalent of
* the componentDidUpdate lifecycle function.
*
* @param {function:function} effect - A useEffect effect.
* @param {array} dependencies - useEffect dependency list.
*/
export const useEffectExceptOnMount = (effect, dependencies) => {
const mounted = React.useRef(false);
React.useEffect(() => {
if (mounted.current) {
const unmount = effect();
return () => {
mounted.current = false;
unmount && unmount();
};
} else {
mounted.current = true;
}
}, dependencies);
};
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