https://www.cnblogs.com/thiaoqueen/p/6904398.html
以下是正确的用法：
var exp = null;
if (!exp && typeof(exp)!=”undefined” && exp!=0)
{
alert(“is null”);
}
记录php学习日志
https://www.cnblogs.com/thiaoqueen/p/6904398.html
var exp = null;
if (!exp && typeof(exp)!=”undefined” && exp!=0)
{
alert(“is null”);
}