Dropdown Selection Change Event in React Js

import * as React from 'react'; 

import styles from './Reactspfx.module.scss'; 

import { IReactspfxProps } from './IReactspfxProps'; 

import { escape } from '@microsoft/sp-lodash-subset';

 export interface IReactGetItemsState

{ selectValue:string }

export default class Reactspfx extends React.Component 

{ 

public constructor(props: IReactspfxProps) { 

super(props); 

this.state = { 

selectValue:"Radish" 

};

}

handleChange = (event) => { this.setState({selectValue:event.target.value}); alert(event.target.value); 

};

public render(): React.ReactElement { alert(this.state.selectValue); return (

  <select value={this.state.selectValue}  onChange={this.handleChange}>
    <option value="Orange">Orange</option>
      <option value="Radish">Radish</option>
      <option value="Cherry">Cherry</option>
    </select>
);

}

SHAREPOINT FRAMEWORK - REACT JS alert on Page Load

Here is my code for getting alert on page load.

componentDidMount() {   
  alert("Hi");
}

public render(): React.ReactElement<IReactCrudProps> {   
    return ( 
        <select id="myDDl">

        </select>
    );
}