//查找id = "time"的元素

var time = document.getElementById("time");

//如果元素为空，往里面插入当前的日期和时间

if(time.firstChild == null){

    time.appendChild(document.createTextNode(new Date().toString()))

}

//修改添加class样式

boxF.style.backgroundColor = "yellow";

//修改class类名/id/内容

boxF.id = "boxUp";

//事件处理

//当用户单击time元素时，更新/改变它的内容

btn.onclick = function(){

        this.innerHTML = new Date().toString()

    }

<div class="reveal">

        <div class="handle">点击</div>

 </div>

       /*class = "reveal"的元素的子元素都不显示*/

        .reveal*{

            display: none;

        }

        /*除了class = "handle"的元素*/

        .reveal*.handle{

            display: none;

        }

//所有的页面逻辑在onload事件之后启动

    window.onload = function(){

        //找到所有class名为"reveal"的容器元素

        var reveal = document.getElementsByClassName("reveal");

        //对每个元素进行遍历

        for(var i = 0;i<reveal.length;++i){

            var let = reveal[i];

            var title = let.getElementsByClassName("handle")[0];

            //当单击这个元素时，呈现剩下的内容

            console.log(title);

            addRevealHandler(title,let)

        }

        function addRevealHandler(title,let){

            console.log(title,let);

            title.onclick = function(){

                if(let.className === "reveal"){

                    let.className = "revealed";

                }

                else if (let.className === "revealed") {

                    let.className = "reveal"

                }

            }

        }

    }