当对象开启时这些

1：null

2：undefined

3：0

4：

它被称为假值

所以你可以这样做

when an object is on of these
1: null
2: undefined
3: 0
4: ""
it is called falsy value
so you can do this

if(!wsdl){
    //do something
}

如果您不确定是否声明了wsdl，那么您可以这样做：

if you are not sure wsdl is declared so you can do this:

if(!window['wsdl']){
    //do something
}