typeof
typeof 'str' // "string"
typeof 123 // "number"
typeof true // "boolean"
typeof undefined // "undefined"
typeof null // "object"
typeof [] // "object"
function test(){}
typeof test // "function"
typeof new Date() // "object"
typeof new RegExp() // "object"
typeof Symbol('val') // "symbol"
```JavaScript String 通过 new创建和直接赋值字符串的区别
## instanceof
instanceof 运算符用来测试一个对象在其原型链中是否存在一个构造函数的 prototype 属性。
var simpleStr = "This is a simple string";
simpleStr instanceof String; // false 检查原型链会找到 undefined
var myString = new String();
myString instanceof String; // true
myString instanceof Object; // true
var newStr = new String("String created with constructor");
newStr instanceof String; // true
newStr instanceof Object; // true
var myDate = new Date();
myDate instanceof Date; // true
myDate instanceof Object; // true
myDate instanceof String; // false
var myObj = {};
myObj instanceof Object; // true
var num = 123
num instanceof Number // false
var myNum = new Number(123)
myNum instanceof Number // true
myNum instanceof Object // true
myNum instanceof Function // false
var bool = true;
bool instanceof Boolean // false
var myBool = new Boolean();
myBool instanceof Boolean // true
myBool instanceof Object // true
var arr = []
arr instanceof Array; // true
arr instanceof Object // true
function test(){}
test instanceof Function; // true
test instanceof Object // true
var date = new Date()
date instanceof Date; // true
date instanceof Object // true
var reg = new RegExp()
reg instanceof RegExp; // true
reg instanceof Object // true
var sym = Symbol('val');
sym instanceof Symbol; // false
sym instanceof Object // false
## Object.prototype.toString.call
var simpleStr = "This is a simple string";
Object.prototype.toString.call(simpleStr) // "[object String]"
var myString = new String();
Object.prototype.toString.call(myString) // "[object String]"
var newStr = new String("String created with constructor");
Object.prototype.toString.call(newStr) // "[object String]"
var myDate = new Date();
Object.prototype.toString.call(myDate) // "[object Date]"
var myObj = {};
Object.prototype.toString.call(myObj) // "[object object]"
var num = 123
Object.prototype.toString.call(num) // "[object Number]"
var myNum = new Number(123)
Object.prototype.toString.call(myNum) // "[object Number]"
var bool = true;
Object.prototype.toString.call(bool) // "[object Boolean]"
var myBool = new Boolean();
Object.prototype.toString.call(myBool) // "[object Boolean]"
var arr = []
Object.prototype.toString.call(arr) // "[object Array]"
function test(){}
Object.prototype.toString.call(test) // "[object Function]"
var date = new Date()
Object.prototype.toString.call(date) // "[object Date]"
var reg = new RegExp()
Object.prototype.toString.call(reg) // "[object RegExp]"
var sym = Symbol('val');
Object.prototype.toString.call(sym) // "[object Symbol]"
