console 命令行工具 X clear
console
<!DOCTYPE html> 
<html>
<head>
    <meta charset="utf-8" />
    <title></title>
<script>
    window.onload=function(){
        var oTxt=document.getElementById("txt");
        var oNum=document.getElementById("num");
        oTxt.onkeyup=function(){

            var str=oTxt.value;
            oNum.innerHTML=str.length
        }
        //键盘属性结束
        var oSearch = document.getElementById("search");
        oSearch.onfocus=function(){
            if(this.value=="百度一下"){
                this.value="";
            }
        }
        oSearch.onblur=function(){
            if(this.value==""){
                this.value="百度一下";
            }
        }
        //鼠标属性：触发焦点和失去焦点结束

    }

</script>


</head>
<body>
    <input id="txt" type="text"/>
    <div>字符串长度为：<span id="num">0</span></div><br>
    <input id="search" type="text" value="百度一下"/>
    <input id="btn" type="button" value="搜索"/>
    
</body>
</html>