Javascript 在React中的类组件开头声明变量时出错

我想知道为什么我把_isMounted=false；myClass开头出现以下错误：SyntaxError:意外标记。我在这个线程中遵循第二个答案：

谢谢你的帮助

这是我的代码，我正在尝试上面的代码来修复以下错误：无法对未安装的组件执行React状态更新。这是一个no-op，但它表示应用程序中存在内存泄漏。要修复此问题，请从以下代码中取消componentWillUnmount方法中的所有订阅和异步任务：

import React, {Component} from 'react'

const months = ["January", "February", "March", "April", "May", "June", "July", "August"," September", "November", "December"]
const days = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"]
const usaTime = new Date().toLocaleString("en-US", {timeZone: "America/New_York"})
const d = new Date(usaTime)

export default class DateTime extends React.Component {
    constructor() {
        super()
        this.state = {
            day: d.getDay(),
            month: d.getMonth(),
            date: d.getDate(),
            year: d.getFullYear(),
            time: d.toLocaleTimeString()
        }
        this.countingSecond = this.countingSecond.bind(this)
    }

    countingSecond() {
        this.setState({
            day: d.getDay(),
            month: d.getMonth(),
            date: d.getDate(),
            year: d.getFullYear(),
            time: d.toLocaleTimeString()
        })
    }

    componentDidMount() {
        setInterval(this.countingSecond, 1000)
    }

    componentWillUnmount(){
        // setInterval(this.countingSecond, 1000)
    }

    render() {
        return(
            <div className="timeclock-main">
                <h5>{days[this.state.day] + ', ' + months[this.state.month] + ' ' + this.state.date + ', ' + this.state.year }</h5>
                <h3>{this.state.time}</h3>
            </div>
            )
    }

}

发件人：

如果您熟悉React类生命周期方法，您可以 将效果挂钩用作componentDidMount、componentDidUpdate和 组件将联合卸载

要检查isMounted=true | | false，可以使用useRef：

编辑-您的工作组件：

const DateTime = (props) =>{

const initialMonths = ["January", "February", "March", "April", "May", "June", "July", "August"," September", "November", "December"]
const initialDays = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"]
const usaTime = new Date().toLocaleString("en-US", {timeZone: "America/New_York"})
const d = new Date(usaTime)

const[days, setDays] = useState(d.getDay())
const[months, setMonths] = useState(d.getMonth())
const[year, setYear] = useState(d.getFullYear())
const[date, setDate] = useState(d.getDate())
const [time, setTime] = useState()

    useEffect(() => { 
        setTime(d.toLocaleTimeString())
        const id = setInterval(() => { setTime(time+1) }, 1000); return () => clearInterval(id); }, [time]
    )
    return (<>
        <h5>{initialDays[days] + ', ' + initialMonths[months] + ' ' + date + ', ' + year }</h5>
        {time}
        </>);
}

export default DateTime;

---

如果我没有使用UseEffect钩子怎么可能？你想实现什么？我有一个显示时间的组件。我在componentDidMount中有一个函数setInterval，我需要修复错误：无法对未安装的组件执行反应状态更新。这是一个no-op，但它表示应用程序中存在内存泄漏。要修复此问题，请取消componentWillUnmount方法中的所有订阅和异步任务。我寻找修复程序，并在这个线程上尝试了第二个答案：无法对未安装的组件执行React状态更新。你明白吗？当我看到代码及其工作原理时，我会更好地理解-然后我就能找到解决方案。你是否尝试添加此项。_ismounted=true；在componentDidMount？中，将东西放在一边，您很少需要这样的东西。你在用什么巴贝尔变换？

const Test = () => {

const componentIsMounted = useRef(true)
useEffect(() => {
    return () => {
        componentIsMounted.current = false
    }
}, [])
    return (
            <div>
                    test
            </div>
    );
}
export default Test;

componentDidMount() { 
  this._ismounted = true;
}

componentWillUnmount() {
   this._ismounted = false;
}

const DateTime = (props) =>{

const initialMonths = ["January", "February", "March", "April", "May", "June", "July", "August"," September", "November", "December"]
const initialDays = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"]
const usaTime = new Date().toLocaleString("en-US", {timeZone: "America/New_York"})
const d = new Date(usaTime)

const[days, setDays] = useState(d.getDay())
const[months, setMonths] = useState(d.getMonth())
const[year, setYear] = useState(d.getFullYear())
const[date, setDate] = useState(d.getDate())
const [time, setTime] = useState()

    useEffect(() => { 
        setTime(d.toLocaleTimeString())
        const id = setInterval(() => { setTime(time+1) }, 1000); return () => clearInterval(id); }, [time]
    )
    return (<>
        <h5>{initialDays[days] + ', ' + initialMonths[months] + ' ' + date + ', ' + year }</h5>
        {time}
        </>);
}

export default DateTime;