How to sync props to state using React hooks : setState()

Question

I am trying to set the state using React hook setState() using the props the component receive. I've tried using the below code:

import React,{useState , useEffect} from 'react';  const Persons = (props) =>  {      // console.log(props.name);     const [nameState , setNameState] = useState(props)     console.log(nameState.name);    console.log(props.name);     return (             <div>                 <p>My name is {props.name} and my age is {props.age}</p>                 <p>My profession is {props.profession}</p>             </div>         )  }  export default Persons; 

The issue is the state is being set upon component being loaded. But when it receive new props, the state is not getting updated. How to update the state in this case? Thanks in advance.

Answer 1

useState hooks function argument is being used only once and not everytime the prop changes. You must make use of useEffect hooks to implement what you would call the componentWillReceiveProps/getDerivedStateFromProps functionality

import React,{useState , useEffect} from 'react';  const Persons = (props) =>  {    const [nameState , setNameState] = useState(props)     useEffect(() => {        setNameState(props);    }, [props])     return (             <div>                 <p>My name is {props.name} and my age is {props.age}</p>                 <p>My profession is {props.profession}</p>             </div>         )  }  export default Persons; 

Answer 2

The props value in useState(props) is used only during the initial render, further state updates are done with the setter setNameState.

In addition, there is no need for useEffect when updating derived state:

const Person = props => {   const [nameState, setNameState] = useState(props.name);   // update derived state conditionally without useEffect   if (props.name !== nameState) setNameState(props.name);   // ... other render code }; 

From React docs:

[...] you can update the state right during rendering. React will re-run the component with updated state immediately after exiting the first render so it wouldn’t be expensive.

[...] an update during rendering is exactly what getDerivedStateFromProps has always been like conceptually.

In essence, we can optimize performance by getting rid of an additional browser repaint phase, as useEffect always runs after the render is committed to the screen.

Working example

This is a contrived example illustrating above pattern - in real code you would read props.name directly. See the React blog post for more appropriate derived state use cases.

const Person = props => {   const [nameState, setNameState] = React.useState(props.name);   // Here, we update derived state without useEffect   if (props.name !== nameState) setNameState(props.name);    return (     <p>       <h3>Person</h3>       <div>{nameState} (from derived state)</div>       <div>{props.name} (from props)</div>       <p>Note: Derived state is synchronized/contains same value as props.name</p>     </p>   ); };  const App = () => {   const [personName, setPersonName] = React.useState("Lui");   const changeName = () => setPersonName(personName === "Lukas" ? "Lui" : "Lukas");    return (     <div>       <Person name={personName} />       <button onClick={changeName}>Change props</button>     </div>   ); };  ReactDOM.render(<App />, document.getElementById("root"));

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