typeof null
// "object"
 
typeof [8]
// "object"
 
typeof {}
// "object"
 
typeof function(){}
// "function"
typeof 2
//"number"
 
typeof ""
//"string"
 
typeof true
//"boolean"
 
typeof undefined
//"undefined"
 
typeof Symbol(2)
// "symbol"

[] instanceof Array
 // true 这种方法可以判断数组，不能区分对象
[] instanceof Object
// true
 
null instanceof Object
// false 也不能区分null

toString.call(function(){})
// "[object Function]"
 
toString.call(null)
//"[object Null]"
 
toString.call([2])
"[object Array]"
 
toString.call(undefined)
//"[object Undefined]"
 
toString.call('stjd')
//"[object String]"
 
toString.call(1)
//"[object Number]"
 
toString.call(true)
//"[object Boolean]"
 
toString.call(Symbol(3))
// "[object Symbol]"
 
toString.call({q:8})
//"[object Object]"

[2].toString()
//"2"

  var obj = {a: 67}
  console.log(obj.toString())
  // [object Array]

console.log(Object.prototype.toString.call([33])) // [object Array]

console.log(Array.prototype.toString.call([2])) //2

  function isType(type) {
   return function(obj) {
    return {}.toString.call(obj) == "[object " + type + "]"
   }
  }
  
  var isObject = isType("Object")
  var isString = isType("String")
  var isArray = Array.isArray || isType("Array")
  var isFunction = isType("Function")
  var isUndefined = isType("Undefined")