Object.prototype.call(變數)可以更詳細地找出變數的型態

console.log(Object.prototype.toString.call(null))
//[object Null]

console.log(Object.prototype.toString.call(123))
//[object Number]

console.log(Object.prototype.toString.call('123'))
//[object String]

console.log(Object.prototype.toString.call(undefined))
//[object Undefined]

console.log(Object.prototype.toString.call(new Date()))
//[object Date]

console.log(Object.prototype.toString.call(function(){}))
//[object Function]

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

console.log(Object.prototype.toString.call(true))
//[object Boolean]

當 typeof 變數，變數本身有宣告但並未賦值，會出現 undefined

var a ;
console.log(typeof a)

//undefined

當變數未宣告且未賦值，會出現 error

console.log(a)

ReferenceError: a is not defined
    at Object.<anonymous> (/Users/rock/.bitnami/stackman/machines/xampp/volumes/root/htdocs/rock/week16-js/type.js:21:13)
    at Module._compile (internal/modules/cjs/loader.js:1158:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1178:10)
    at Module.load (internal/modules/cjs/loader.js:1002:32)
    at Function.Module._load (internal/modules/cjs/loader.js:901:14)
    at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:74:12)
    at internal/main/run_main_module.js:18:47