JS中如何判断一个字符串是存在的还是非空的

  if(typeof unknownVariable ==='string' && unknownVariable.length){
        ...
    }

 let str1="test"
    let str2=new String('test2')
    let arr1=[1,2,3]
    let fn1=function(){
        console.log('this is a function')
    }
    let obj1={
        name:'gpd'
    }
    let obj2=new Object()
    obj2.name='gpd'   
    str1.valueOf()  // "test"
    str2.valueOf()  //"test2"
    arr1.valueOf()  //[1,2,3]
    fn1.valueOf()  //fn1(){}
    obj1.valueOf() // {name:"gpd"}
    obj2.valueOf() // {name:"gpd"}

if(typeof unknownVariable !=undefined 
    && unknownVariable 
   && typeof unknowVariable.valueOf() === "string")
   {
       ...
   }