js类型检测

typeof
适用于基本类型和函数的检测 遇到null无效

• typeof 100 -> "number"
• typeof true -> "boolean"
• typeof function -> "function"
• typeof (undefined) -> "undefined"
• typeof new Object() -> "object"
• typeof [1,2] -> "object"
• typeof NaN -> "number"
• typeof null -> "object"

instanceof obj instanceof Object
适合自定义对象，也可用来检测原生对象，在不同iframe和window间检测时失效 判断左边的操作数对象的原型链上是否有右侧的构造函数的属性

• [1,2] instanceof Array === true
• new Object() instanceof Array === false

function Person(){}  //undefined
function Student(){} //undefined
Student.prototype = new Person()  //Person
Student.prototype.constructor = Student // function Student(){}
var bosn = new Student() //undefined
bosn instanceof Student //true
var one = new Person() //undefined
one instanceof Person //true
one instanceof Student // false
bosn instanceof Person //true

Object.prototype.toString
适合内置对象和基元类型，遇到null和undefined失效

• Object.prototype.toString.apply([]) === "[object Array]";
• Object.prototype.toString.apply(function(){}]) === "[object Function]";
• Object.prototype.toString.apply(null) === "[object Null]";
• Object.prototype.toString.apply(undefined) === "[object Undefined]"; 注：IE6/7/8 Object.prototype.toString.apply(null) === "[object Object]"

constructor 继承自己原型

duck type