数据类型检测

1.1 typeof

typeof操作符返回一个字符串，表示未经计算的操作数的类型；

1.2 instanceof

var str = "This is a simple string"; 
var num = 1111;
var boolean = true;
var und = undefined;
var nl = null;
var sb = Symbol('1111');
var obj = {}; // 非原始类型数据字面量定义

console.log(str instanceof String);         // false
console.log(num instanceof Number);         // false
console.log(boolean instanceof Boolean);    // false
console.log(nl instanceof Object);          // false
console.log(sb instanceof Symbol);          // false
console.log(obj instanceof Object);         // true

var strN = new String("This is a simple string");
var numN = new Number(1111);
var booleanN = new Boolean(true);
var objN = new Object();

console.log(strN instanceof String);            // true
console.log(numN instanceof Number);            // true
console.log(booleanN instanceof Boolean);       // true
console.log(objN instanceof Object);            // true

1.3 Object.propotype.toString

Object.prototype.toString.call('string');       //"[object String]"
Object.prototype.toString.call(1111);           //"[object Number]"
Object.prototype.toString.call(true);           //"[object Boolean]"
Object.prototype.toString.call(null);           //"[object Null]"
Object.prototype.toString.call(undefined);      //"[object Undefined]"
Object.prototype.toString.call(Symbol('111'));  //"[object Symbol]"
Object.prototype.toString.call({});             //"[object Object]"

1.4 constructor

比较对象的构造函数与类的构造函数是否相等

var a = {}
a.constructor === Object     // true

var b = '111';
b.constructor === String    // true

var strN = new String('11111');
strN.constructor === String // true

var c = true;
c.constructor === Boolean   // true

var d = Symbol('symbol')
d.constructor === Symbol    // true

1.5 propotype

比较对象的原型与构造函数的原型是否相等

var a = {}
a.__proto__ === Object.prototype     // true

var t = new Date();
t.__proto__ === Date.prototype      // true


var str = 'sting';
str.__proto__ === String.prototype  // true

var strN = new String('11111');
strN.__proto__ === String.prototype // true