//方式一：最常見的方式，通過數(shù)字字面量方式聲明 var num = 123; //方式二：偶爾使用方式，大部分情況下是將字符串轉(zhuǎn)成數(shù)字 var num = Number(123); //方式三：很少使用，各神書，包括犀牛書，都將其列入不推薦方式 var num = new Number(123); //方式四：神方式，目前還沒見過人使用 var num = new Object(123); //方式五：更離奇，更詭異 var num = Object(123);
可以看到，在上5種聲明方式種，方式一不用怎么說了，平常都是這樣用的；方式三 to 方式五屬于比較的使用，下文會分別說明： 1.五種聲明方式的區(qū)別？當你用顫巍巍的手指敲下代碼后，究竟發(fā)生了神馬？ 2.方式一聲明的明明不是對象，但為什么平常我們可以在上面調(diào)用方法，如toString等？
二、各種聲明方式之間的區(qū)別 方式一：var num = 123; EC5說明： A numeric literal stands for a value of the Number type. This value is determined in two steps: first, a mathematical value (MV) is derived from the literal; second, this mathematical value is rounded as described below //.....個人總結(jié)摘要： 1.解析變量的值，比如說取出整數(shù)部分、小數(shù)部分等，因為數(shù)字聲明方式還可以為num = .123，num = 123e4等形式 2.對解析出來的值取近似值，比如num = 123.33333333333333...3333333333333333333333333....，這個時候就要取近似值了，具體取近似則規(guī)則不展開 3.此種方式聲明的變量，只是個簡單的數(shù)字字面量，并不是對象（至于為什么可以在上面調(diào)用toString等方法，后文講解） 方式二：var num = Number(123); EC5說明： 15.7.1 The Number Constructor Called as a Function When Number is called as a function rather than as a constructor, it performs a type conversion. 15.7.1.1 Number ( [ value ] ) Returns a Number value (not a Number object) computed by ToNumber(value) if value was supplied, else returns +0.個人總結(jié)摘要： 1.此處只是將Number當作一個普通的函數(shù)來調(diào)用，而不是構(gòu)造方法，因此返回的不是對象，而是一個簡單的數(shù)值 2.本質(zhì)與方式一相同；相對于方式一的區(qū)別在于，需要針對傳入?yún)?shù)的類型，執(zhí)行不同的類型轉(zhuǎn)換過程，試圖將參數(shù)解析成對應(yīng)的數(shù)值，具體規(guī)則如下： 方式三：var num = new Number(123); EC5說明： 15.7.2 The Number Constructor When Number is called as part of a new expression it is a constructor: it initialises the newly created object. 15.7.2.1 new Number ( [ value ] ) The [[Prototype]] internal property of the newly constructed object is set to the original Number prototype object, the one that is the initial value of Number.prototype (15.7.3.1). The [[Class]] internal property of the newly constructed object is set to "Number". The [[PrimitiveValue]] internal property of the newly constructed object is set to ToNumber(value) if value was supplied, else to +0. The [[Extensible]] internal property of the newly constructed object is set to true.個人總結(jié)摘要： 1.此處將Number作用構(gòu)造方法調(diào)用，返回的是Number類型的對象，該對象能夠訪問Number的原型屬性以及方法；這樣說可能有些迷惑，后面會說到
方式四：var num = new Object(123); EC5說明： 15.2.2 The Object Constructor When Object is called as part of a new expression, it is a constructor that may create an object. 15.2.2.1 new Object ( [ value ] ) When the Object constructor is called with no arguments or with one argument value, the following steps are taken: 1.If value is supplied, then a. If Type(value) is Object, then 1.If the value is a native ECMAScript object, do not create a new object but simply return value. 2.If the value is a host object, then actions are taken and a result is returned in an implementation-dependent manner that may depend on the host object. b. If Type(value) is String, return ToObject(value). c. If Type(value) is Boolean, return ToObject(value). d. If Type(value) is Number, return ToObject(value). 2.Assert: The argument value was not supplied or its type was Null or Undefined. 3.Let obj be a newly created native ECMAScript object. 4.Set the [[Prototype]] internal property of obj to the standard built-in Object prototype object (15.2.4). 5.Set the [[Class]] internal property of obj to "Object". 6.Set the [[Extensible]] internal property of obj to true. 7.Set all the internal methods of obj as specified in 8.12. 8.Return obj. 個人理解說明： 上面洋洋灑灑的貼了好多文字，可能看著有些頭疼，因為通過 new Object(param) 這種方式聲明變量，根據(jù)傳入?yún)?shù)具體類型的不同，得到的結(jié)果也會有區(qū)別，比如說數(shù)據(jù)類型。這里面具體轉(zhuǎn)換的規(guī)則，可以忽略不計，我們只看我們目前關(guān)系的部分，即上面標紅色的文字，要點有： 1.傳遞了參數(shù)，且參數(shù)是一個數(shù)字，則創(chuàng)建并返回一個Number類型的對象 ―― 沒錯，其實等同于方式三 2.該Number對象的值等于傳入的參數(shù)，內(nèi)部的[[prototype]]屬性指向Number.prototype 方式五：var num = Object(123); EC5說明： 15.2.1 The Object Constructor Called as a Function When Object is called as a function rather than as a constructor, it performs a type conversion. 15.2.1.1 Object ( [ value ] ) When the Object function is called with no arguments or with one argument value, the following steps are taken: 1.If value is null, undefined or not supplied, create and return a new Object object exactly as if the standard built-in Object constructor had been called with the same arguments (15.2.2.1). 2.Return ToObject(value). 個人理解說明： 1.當傳入的參數(shù)為空、undefined或null時，等同于 new Object(param)，param為用戶傳入的參數(shù) 2.否則，返回一個對象，至于具體轉(zhuǎn)換成的對象類型，可參見下表；具體到上面的例子，本質(zhì)等同于new Number(123)： 3. 簡單測試用例
