Javascript類型檢測

1.typeof

typeof 適合基本類型及function檢測，遇到null失效。

typeof 100; // "number"
typeof true; // "boolean"
typeof fun(); // "function"
typeof NaN; // "number"
typeof undefined; // "undefined"
typeof null; // "object"
typeof new Object(); // "object"
typeof [1,2]; // "object"

2.instanceof

instanceof一般用於對象類型的判斷，是基於原型鏈的判斷。

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

function Student(){}
function Teacher(){}

var s = new Student()

console.log(s instanceof Student) // true
console.log(s instanceof Teacher) // false

Student.prototype = new Teacher()
var s2 = new Student()
console.log(s2 instanceof Student) // true
console.log(s2 instanceof Teacher) // true

PS:適合自定義對象，也可以用來檢測原生對象。不同window或iframe間的對象類型檢測不能用instanceof。

3.Object.prototype.toString.apply()

通過{}.toString拿到，適合內置對象和基本類型，遇到null和undefined失效（IE678返回[object Object]）。

Object.prototype.toString.apply([]) === "[object Array]"
Object.prototype.toString.apply(function(){}) === "[object Function]"
Object.prototype.toString.apply(null) === "[object Null]" // IE6/7/8返回"[object Object]"
Object.prototype.toString.apply(undefined) === "[object Undefined]"
Object.prototype.toString.apply(123) === "[object Number]"
 Object.prototype.toString.apply('123') === "[object String]"
 Object.prototype.toString.apply(true) === "[object Boolean]"

 console.log(Object.prototype.toString.apply(new Object())) //[object Object]
function Student(){}
var s = new Student()
console.log(Object.prototype.toString.apply(s)) //[object Object]

console.log(Object.prototype.toString.apply(window)) // [object Window]
console.log(Object.prototype.toString.apply(new Date())) //[object Date]