JS中判断字符串存在和非空的方法

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  ()
  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")
  {
    ...
  }