前面的話

　　javascript有複雜的型別系統，型別識別則是基本的功能。javascript總共提供了四種類型識別的方法，本文將對這四種方法進行詳細說明

typeof運算子

　　typeof是一元運算子，放在單個運算元的前面，返回值為表示運算元型別的首字母小寫的字串

　　[注意]typeof運算子後面帶不帶圓括號都可以

console.log(typeof 'a');//'string'
console.log(typeof ('a'));//'string'

識別

　　【1】可以識別標準型別(將Null識別為'object')　

　　【2】不能識別具體的物件型別(Function除外)

console.log(typeof "jerry");//"string"
console.log(typeof 12);//"number"
console.log(typeof true);//"boolean"
console.log(typeof undefined);//"undefined"
console.log(typeof null);//"object"
console.log(typeof {name: "jerry"});//"object"

console.log(typeof function(){});//"function"
console.log(typeof
 
 []);//"object"
console.log(typeof new Date);//"object"
console.log(typeof /\d/);//"object"
function Person(){};
console.log(typeof new Person);//"object"

　　[注意]判斷一個值是否為null型別的最佳方法是直接和null進行恆等比較

console.log(typeof null);//'object'
console.log(null === null);//true
console.log(undefined === null);//false
 

console.log('null' === null);//false

instanceof運算子

　　instanceof是一個二元運算子，左運算元是一個物件，右運算元是一個建構函式。如果左側的物件是右側建構函式的例項物件，則表示式返回true；否則返回false

　　如果左運算元不是物件，返回false，如果右運算元不是函式，則丟擲一個型別錯誤異常TypeError

console.log(123 instanceof function(){});//false
//Uncaught TypeError: Right-hand side of 'instanceof' is not an object
console.log({} instanceof 123);

[注意]所有的物件都是Object的例項

識別

　　【1】可以識別內建物件型別、自定義型別及其父型別

　　【2】不能識別標準型別，會返回false

　　【3】不能識別undefined、null，會報錯

console.log("jerry" instanceof String);//false
console.log(12 instanceof Number);//false
console.log(true instanceof Boolean);//false
console.log(undefined instanceof Undefined);//報錯
console.log(null instanceof Null);//報錯
console.log({name: "jerry"} instanceof Object);//true

console.log(function(){} instanceof Function);//true
console.log([] instanceof Array);//true
console.log(new Date instanceof Date);//true
console.log(/\d/ instanceof RegExp);//true
function Person(){};
console.log(new Person instanceof Person);//true
console.log(new Person instanceof Object);//true

constructor屬性

　　例項物件的constructor屬性指向其建構函式。如果是內建型別，則輸出function 資料型別(){[native code]}；如果是自定義型別，則輸出function 資料型別(){}

識別

　　【1】可以識別標準型別、內建物件型別及自定義型別

　　【2】不能識別undefined、null，會報錯，因為它倆沒有建構函式

console.log(("jerry").constructor);//function String(){[native code]}
console.log((12).constructor);//function Number(){[native code]}
console.log((true).constructor);//function Boolean(){[native code]}
console.log((undefined).constructor);//報錯
console.log((null).constructor);//報錯
console.log(({name: "jerry"}).constructor);//function Object(){[native code]}

console.log((function(){}).constructor);//function Function(){[native code]}
console.log(([]).constructor);//function Array(){[native code]}
console.log((new Date).constructor);//function Date(){[native code]}
console.log((/\d/).constructor);//function RegExp(){[native code]}
function Person(){};
console.log((new Person).constructor);//function Person(){}

可以將constructor屬性封裝成一個型別識別方法

function type(obj){
    var temp = obj.constructor.toString();
    return temp.replace(/^function (\w+)\(\).+$/,'$1');
}

function type(obj){
    var temp = obj.constructor.toString().toLowerCase();
    return temp.replace(/^function (\w+)\(\).+$/,'$1');
}
console.log(type("jerry"));//"string"
console.log(type(12));//"number"
console.log(type(true));//"boolean"
console.log(type(undefined));//錯誤
console.log(type(null));//錯誤
console.log(type({name: "jerry"}));//"object"

console.log(type(function(){}));//"function"
console.log(type([]));//"array"
console.log(type(new Date));//"date"
console.log(type(/\d/));//"regexp"
function Person(){};
console.log(type(new Person));//"person"

Object.prototype.toString()方法

　　物件的類屬性是一個字串，用以表示物件的型別資訊。javascript沒有提供設定這個屬性的方法，但有一種間接方法可以查詢它。Object.prototype.toString()方法返回瞭如下格式的字串：[object 資料型別]

識別

　　【1】可以識別標準型別及內建物件型別

　　【2】不能識別自定義型別

console.log(Object.prototype.toString.call("jerry"));//[object String]
console.log(Object.prototype.toString.call(12));//[object Number]
console.log(Object.prototype.toString.call(true));//[object Boolean]
console.log(Object.prototype.toString.call(undefined));//[object Undefined]
console.log(Object.prototype.toString.call(null));//[object Null]
console.log(Object.prototype.toString.call({name: "jerry"}));//[object Object]

console.log(Object.prototype.toString.call(function(){}));//[object Function]
console.log(Object.prototype.toString.call([]));//[object Array]
console.log(Object.prototype.toString.call(new Date));//[object Date]
console.log(Object.prototype.toString.call(/\d/));//[object RegExp]
function Person(){};
console.log(Object.prototype.toString.call(new Person));//[object Object]

　　可以將Object.prototype.toString()方法封裝成一個型別識別方法

function type(obj){
    return Object.prototype.toString.call(obj).slice(8,-1).toLowerCase();
}
console.log(type("jerry"));//"string"
console.log(type(12));//"number"
console.log(type(true));//"boolean"
console.log(type(undefined));//"undefined"
console.log(type(null));//"null"
console.log(type({name: "jerry"}));//"object"

console.log(type(function(){}));//"function"
console.log(type([]));//"array"
console.log(type(new Date));//"date"
console.log(type(/\d/));//"regexp"
function Person(){};
console.log(type(new Person));//"object"

　　[注意]如果是包裝物件，Object.prototype.toString()方法將返回其原始型別

console.log(Object.prototype.toString.call(new Number(123)));//[object Number]
console.log(Object.prototype.toString.call(123));//[object Number]
console.log(Object.prototype.toString.call(new String('abc')));//[object String]
console.log(Object.prototype.toString.call('abc'));//[object String]
console.log(Object.prototype.toString.call(new Boolean(true)));//[object Boolean]
console.log(Object.prototype.toString.call(true));//[object Boolean]