JS类型判断

  如何检测对象类型？可能第一反应是使用 typeof。但是 typeof 真的好用吗？不见得，如 typeof null 返回的是 “object”。 还有引用类型（Function除外）也返回的是 object。除了 typeof 以外还可以使用 Object.prototype.toString.call()、constructor、 和 instanceof。

1、typeof

var a = 1;
typeof a  => "number"
typeof 12 => "number"

var b = NaN;
typeof b => "number"
typeof NaN => "number"

var c = "SD";
typeof c => "string"
typeof "SDF" => "string"

var d = true;
typeof d => "boolean"
typeof false => "boolean"

var e;
typeof e => "undefined"
typeof undefined => "undefined"

var f = null;   // 注意
typeof f => "object"
typeof null => "object"

var g = new Object;
typeof g => "object"
typeof {} => "object"

var h = Array("s","v")；
typeof h => "object"
typeof [] => "object"

var i = function(){}
typeof i => "function"

var j = new i();
typeof j => "object"

var k = /123/   //正则
typeof k => "object"

注意：

• typeof（引用类型）除了函数，其他都是 ‘object’
• typeof null 为 ‘object’

2、Object.prototype.toString.call(obj) 或 {}.toString.call(obj)

Object.prototype.toString.call(1)  => "[object Number]"
Object.prototype.toString.call(NaN) => "[object Number]"
Object.prototype.toString.call("as") => "[object String]"
Object.prototype.toString.call(true) => "[object Boolean]"
Object.prototype.toString.call(function(){}) => "[object Function]"
Object.prototype.toString.call(undefined) => "[object Undefined]"
Object.prototype.toString.call(null) => "[object Null]"
Object.prototype.toString.call({}) => "[object Object]"
Object.prototype.toString.call([]) => "[object Array]"
Object.prototype.toString.call(new Date()) => "[object Date]"
Object.prototype.toString.call(new Error()) => "[object Error]"
Object.prototype.toString.call(/123/) => "[object RegExp]"

function I(){}
var i = new I()
Object.prototype.toString.call(i) => "[object Object]"

可以看到Object.prototype.toString.call()可以将内置类型准确判断出来。
Object.prototype.toString方法原理请参考：JavaScript中Object.prototype.toString方法的原理一文。

3、instanceof 和 constructor

• instanceof：用于判断一个变量是否为某个对象的实例

• constructor：用于判断一个对象的原型，constructor 属性返回对创建此对象的数组函数的引用。

  var a = 123
  a instanceof Number => false
  a.constructor == Number  => true
  
  var b = "sdf"
  b instanceof String  => false
  b.constructor == String => true
  
  var c = true
  c instanceof Boolean => false
  c.constructor == Boolean => true
  
  var d = null
  d instanceof Object => false
  d.constructor == null => 报错，Cannot read property 'constructor' of null
  
  var e = undefined
  e instanceof undefined  => 报错，Right-hand side of 'instanceof' is not an object
  e.constructor == undefined => 报错，Cannot read property 'constructor' of undefined
  
  var f = function(){}
  e instanceof Function => true
  e.constructor == Function => true
  
  var g = []
  g instanceof Array => true
  g.constructor == Array => true
  
  var h = {
      "a": 1,
      "b": 2
  }
  h instanceof Object => true
  h.constructor == Object => true
  
  function I(){}
  var i = new I()
  i instanceof I => true
  i.constructor == I => true