react-倒计时30s的 实现

this.state={
  time:''
}
countDown = (waitTime) => {
        if (waitTime > 0) {
            waitTime--;
           this.setState({
               time:waitTime
           })
        } else {
          //  history.push('/Login')
          //  return;
        }
        setTimeout(() => {
            this.countDown();
        }, 1000);
	}
 submit = () => {
    this.setState({ submit_visible: true });
    this.countDown(30);
  };

字符串反转

let str='abcdefg'
 const test=(str)=>{
    return str.split('').reverse().join('')
    }
 console.log(test('abcdefg'))