javascript - 如何确定变量是“未定义”还是“空”？(How to determine if variable is 'undefined' or 'null'?)

Question

How do I determine if variable is undefined or null ?(如何确定变量是否undefined或为null ？)

My code is as follows:(我的代码如下：) var EmpName = $("div#esd-names div#name").attr('class'); if(EmpName == 'undefined'){ //DO SOMETHING }; <div id="esd-names"> <div id="name"></div> </div> But if I do this, the JavaScript interpreter halts execution.(但是，如果执行此操作，JavaScript解释器将停止执行。)   ask by sadmicrowave translate from so

Answer 1

You can use the qualities of the abstract equality operator to do this:(您可以使用抽象相等运算符的品质来做到这一点：)

if (variable == null){ // your code here. } Because null == undefined is true, the above code will catch both null and undefined .(因为null == undefined为true，所以上面的代码将捕获null和undefined 。)