为什么报错Failed to compile？？sos

import React from 'react'

class CommentBox extends React.Component{

    constructor(props){

        super(props)

        this.state={

            value:''

        }

        this.handleChange=this.handleChange.bind(this)

        this.handleSubmit=this.handleSubmit.bind(this)

    }

    handleChange(event){

       this.setState({

           value: event.target.value

       })

    }

    handleSubmit(event){

        this.setState({

            alert(this.state.value)

            event.preventDefault()

        })

    }

    render(){

        return(

            <form className="p-5" onSubmit={this.handleSubmit}>

                <div className="form-group">

                    <label>留言内容</label>

                    <input type="text"  className="form-control" placeholder="请输入内容" value={this.state.value} onChange={this.handleChange}/>

                </div>

                <button type="submit" className="btn btn-primary">留言</button>

            </form>

        )

    }

}

export default CommentBox

报错：

Failed to compile

./src/components/CommentBox.js
  Line 18:  Parsing error: Unexpected keyword 'this'

  16 |     handleSubmit(event){
  17 |         this.setState({
> 18 |             alert(this.state.value)
     |                   ^
  19 |             event.preventDefault()
  20 |         })
  21 |     }