How to initialize the react functional component state from props - reactjs

Question

I'm using React hooks for app state, I wondered about how to initialize the functional component state using props? The useState hook doc says something definitive like,
const [count, setCount] = useState(0);
I want to initialize that 0 value by the value of props being passed to the component. The Older as,
import React from 'react';
export default class Sym extends React.Component{
constructor(props){
super(props);
this.state = {
sym : [0,3,2,8,5,4,1,6],
active: this.props.activeSym
}
this.setActive = this.setActive.bind(this);
}
setActive(itemIndex){
this.setState({
active: itemIndex
});
}
render(){
return (
<div><h1>{ this.state.sym[this.state.active]}</h1></div>
);
}
}
works fine. Where the parent Component passes activeSym prop and Sym component initializes the state with it using this.props.activeSym in constructor. Is there any workaround to achieve same in function component?

Answer 1

First you can define it from props (if the prop exist):
const [count, setCount] = useState(activeSym);
And then you can update this value, when prop doesn't have a value immediately (when component rendered):
useEffect(() => {
if (activeSym) {
setCount(activeSym);
}
}, [activeSym])

Answer 2

Yes, this can be possible with functional component too! You just need to add useEffect to listen to prop change for initializing state with prop value
export const newComponent = (props) => {
const { path, value, info, update } = props;
const [val, setVal] = useState(value);
useEffect(() => {
setVal(value);
}, [value]);
return <div>{val}</div>;
};
Attching sandbox link
https://codesandbox.io/s/confident-agnesi-ohkq7?file=/src/MakeComponent.js

Answer 3

Yes you can first define state using props:
const [name, setName] = useState(props.obj?.name);
And then you can if the state is still undefined means props doesn't have a value, then:
useEffect(() => {
if (JSON.stringify(props.obj) !== "{}") {
setName(props.obj?.name);
}
}, [props.obj])

Answer 4

Just as follows :
const MyFunctionalComponent = ({myProp}) => {
const [count, setCount] = useState(myProp)
return (
/* ... */
)
}

Answer 5

There are two ways you can change the state:
one is using this.state and
another one is this.setState.
We use the first method to initialize the state in the constructor, and the second method is used for the rest of the time.
Initialize State in the Constructor
One way is to initialize the state is in the constructor. As we discussed earlier constructor is the first method to be called when React instantiates the class. This is the perfect place to initialize the state for the component because the constructor is called before the React renders the component in the UI.
class WithConstructor {
constructor() {
this.state = {
name: "StackOverflow"
}
}
}
Initialize State Without Constructor
Another way of initializing state in React is to use the Class property. Once the class is instantiated in the memory all the properties of the class are created so that we can read these properties in the render function.
class WithoutConstructor {
state = {
name: "StackOverflow"
}
}