The screen is not updating

user_14491004 0
Viewed 44

i want my state to update every 1sec but its not working.
when I console logged this second, it works OK, but the screen is not updated
below is my codes

import React, { PureComponent } from 'react';

const date = new Date()
let seconds=date.getSeconds()

class Timer extends PureComponent {
    state={
        seconds:seconds,
    }

    changeSecond=()=>{
        this.setState((prevState)=>{
            this.state.seconds=prevState.seconds+1
        })
    }

    componentDidMount(){
        this.interval=setInterval(this.changeSecond,1000)
    }

    componentWillUnmount(){
        clearInterval(this.interval)
    }

    render() {
        return (
            <>
                <div>{this.state.seconds}초</div>
            </>
        )
    }
};

reactjs timer javascript
2 Answers

Looks like an invalid state update. It should probably be:

changeSecond = () => {
  this.setState((prevState)=> ({ seconds: prevState.seconds + 1 }));
}

Update from the previous state and return a new state object with the correct object shape.

user_86908570

You're updating state incorrectly. The callback form of the state setter must return the new state:

changeSecond = () => {
    this.setState((prevState) =>
        ({seconds: prevState.seconds + 1})
    );
};

Live Example:

const { PureComponent } = React;

const date = new Date();
let seconds = date.getSeconds();

class Timer extends PureComponent {
    state={
        seconds:seconds,
    };

    changeSecond = () => {
        this.setState((prevState) =>
            ({seconds: prevState.seconds + 1})
        );
    };

    componentDidMount() {
        this.interval=setInterval(this.changeSecond,1000);
    }

    componentWillUnmount() {
        clearInterval(this.interval);
    }

    render() {
        return (
            <React.Fragment>
                <div>{this.state.seconds}초</div>
            </React.Fragment>
        );
    }
};

ReactDOM.render(<Timer />, document.getElementById("root"));
<div id="root"></div>

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.13.0/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.13.0/umd/react-dom.production.min.js"></script>

user_1572470
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