js变量值""和Undefined 和 Null形式、js判断字符串是否为""或null

 js变量值""和Undefined 和 Null

var person;
document.write(person + "<br>");//undefined
document.write((person==null) + "<br>");//true
    
var car="Volvo";
document.write(car + "<br>");//Volvo
document.write((car==null) + "<br>");//false
    
var three=null;
document.write(three + "<br>");//null
document.write((three==null) +"<br>");//true

var four="";
document.write(four + "<br>");//控制台输出什么都没有
document.write((four=="") +"<br>");//true
document.write((four==null) +"<br>");//false
<!DOCTYPE html>
<html>
<head> 
<meta charset="utf-8"> 
<title>菜鸟教程(runoob.com)</title> 
</head>
<body>

<script>
var person;
var car="Volvo";
var three=null;
var four="";
document.write(person + "<br>");
document.write((person==null) + "<br>");
document.write("------------------------<br>");
document.write(car + "<br>");
document.write((car==null) + "<br>");
document.write("------------------------<br>");
document.write(three + "<br>");
document.write((three==null) +"<br>");
document.write("------------------------<br>");    
document.write(four + "<br>");
document.write((four=="") +"<br>");
document.write((four==null) +"<br>");


</script>

</body>
</html>
View Code

测试：https://www.runoob.com/try/try.php?filename=tryjs_undefined

 js判断字符串是否为""或null

console.error(roomid);  //roomid=null时
console.error(roomid != 'null');

roomid = ""; //roomid=""时
console.error(roomid != "");

js中判断变量是否=“”和是否=null

if (roomid != "" && roomid != 'null') {
    $("input[name=roomid]").val(roomid);
}

 

posted @ 2023-03-14 10:50  じ逐梦  阅读(82)  评论(0)    收藏  举报