详解JavaScript中的数据类型，以及检测数据类型的

let s = Symbol()
console.log(typeof s) // "symbol"

let s1 = Symbol('s1')
let s2 = Symbol('s2')
console.log(s1) // Symbol(s1)
console.log(s2) // Symbol(s2)

console.log(typeof 1) // "number"
console.log(typeof 'a') // "string"
console.log(typeof undefined) // "undefined"
console.log(typeof true) // "boolean"
console.log(typeof null) // "object"
console.log(typeof ) // "symbol"
function fun(){ }
console.log(typeof fun) // "function"

let fun = function(){ }
fun instanceof Function  //true
let obj ={ }
obj instanceof Object //true
let arr = [ ]
arr instanceof Array //true

1 instanceof Number //false 
null instanceof Object // false

Object.prototype.toString.call([])
// "object Array"
Object.prototype.toString.call(1)
// "object Number"
Object.prototype.toString.call(null)
// "object Null"
Object.prototype.toString.call(undefined)
// "object Undefined"
Object.prototype.toString.call({})
// "object Object"
Object.prototype.toString.call(function add(){})
// "object Function"
....

let arr =[]
arr.constructor==Array
// true
let fun = function(){}
fun.constructor==Function
//true