摘要：文章部分實(shí)例和內(nèi)容來自鳥哥的作用域原理首先應(yīng)該注意幾個(gè)點(diǎn)函數(shù)也是對(duì)象因?yàn)槭窃谌肿饔糜虍?dāng)中當(dāng)執(zhí)行到時(shí)即會(huì)產(chǎn)生在當(dāng)中，函數(shù)的運(yùn)行是在它被定義的作用域當(dāng)中，而非執(zhí)行的作用域當(dāng)中。

文章部分實(shí)例和內(nèi)容來自鳥哥的blogJavascript作用域原理

首先應(yīng)該注意幾個(gè)點(diǎn)：

函數(shù)也是對(duì)象

variable object(VO)

A variable object is a container of data associated with the execution context. It’s a special object that stores variables and function declarations defined in the context.

    var foo=10;
    function func(){};
    
    //因?yàn)槭窃谌肿饔糜虍?dāng)中,so...
    Global VO={
        foo:10,
        func:
    }

Activation Object(AO)

When a function is activated (called) by the caller, a special object, called an activation object is created.

It’s filled with formal parameters and the special arguments object (which is a map of formal parameters but with index-properties). The activation object then is used as a variable object of the function context.

A function’s variable object is the same simple variable object, but besides variables and function declarations, it also stores formal parameters and arguments object and called the activation object.

    function foo(x,y){
        var z=30;
        function bar(){};
    }
    foo(10,20);
    //當(dāng)執(zhí)行到foo(10,20)時(shí)即會(huì)產(chǎn)生AO
    Activation Object={
        z:30,
        x:10,
        y:20,
        bar:,
        arguments:{0:10,1:20,length:2}
    }

Scope Chain(Scope Chain=Activation Object + [[scope]])

A scope chain is a list of objects that are searched for identifiers appear in the code of the context.

在JavaScript當(dāng)中，函數(shù)的運(yùn)行是在它被定義的作用域當(dāng)中，而非執(zhí)行的作用域當(dāng)中。

先看一段代碼：

    var name="laurence?";
    
    function show(){
        console.log(name);
        var name="laurence?";
        console.log(name);
    }

最后輸出： undefined  laurence?



Object={
    name1:undefined,//第一個(gè)輸出為undefined,表達(dá)式聲明的局部變量覆蓋全局變量，函數(shù)執(zhí)行到這一語句的時(shí)候才進(jìn)行賦值操作name="laurence?"，之前name=undefined
    name2:"laurence?",//第二個(gè)輸出為"laurence?"
}
window={
    name:"laurence?",
    show:function()
}
    

注意點(diǎn)：

函數(shù)也是對(duì)象

變量提升

函數(shù)在定義過程中，會(huì)將定義時(shí)刻的scope chain鏈接到這個(gè)函數(shù)對(duì)象的[[scope]]屬性上，這個(gè)屬性包含了函數(shù)被創(chuàng)建的作用域中 對(duì)象 的集合，同時(shí)它的作用域會(huì)被創(chuàng)建此函數(shù)的作用域中可訪問的數(shù)據(jù)對(duì)象填充。(對(duì)象的集合、對(duì)象鏈)

函數(shù)的執(zhí)行過程中，會(huì)創(chuàng)建一個(gè) 活動(dòng)對(duì)象 (activation object)，該對(duì)象包含了所有的局部變量、命名參數(shù)、參數(shù)集合和this，然后將這個(gè)活動(dòng)對(duì)象作為此時(shí)作用域鏈的最前端，每個(gè)活動(dòng)對(duì)象都有自己的作用域鏈，用于標(biāo)識(shí)符的解析，當(dāng)活動(dòng)對(duì)象被創(chuàng)建時(shí)，而它的作用域初始化為當(dāng)前運(yùn)行函數(shù)的[[scope]]所包含的對(duì)象。

var func=function(lps,rps){
    var name="XL";
    ....
}
func();

var func=function(){ } 相當(dāng)于匿名函數(shù)的執(zhí)行

(在執(zhí)行函數(shù)創(chuàng)建活動(dòng)對(duì)象（Obj）的過程中，會(huì)創(chuàng)建一個(gè)arguments屬性，然后會(huì)給這個(gè)活動(dòng)對(duì)象添加2個(gè)屬性名，Obj.lps,Obj.rps對(duì)于每一個(gè)在這個(gè)函數(shù)中申明的局部變量和函數(shù)定義，都作為該活動(dòng)對(duì)象的同名屬性，然后將調(diào)用參數(shù)賦值給形參，對(duì)于缺少的調(diào)用參數(shù)，賦值為undefined)

//這里func()執(zhí)行時(shí)
Obj(AO)={
    lps:undefined,
    rps:undefined,
    arguments:{}
    name:"XL"
}
//創(chuàng)建func()時(shí)為全局對(duì)象:
window(Global VO)={
    func:function()
}

實(shí)際的例子:

function factory(){
    var name="laruence";
    var intro=function(){
        console.log("I"m "+name);
    }
    return intro;
}
function app(para){
    var name=para;
    var func=factory();
    func();
}

app("eve");

當(dāng)調(diào)用app的時(shí)候，scope chain是由{window活動(dòng)對(duì)象(全局)}+{app活動(dòng)對(duì)象}組成

此時(shí)的[[scope chain]]為( 可訪問的數(shù)據(jù)對(duì)象訪問 )：

[[scope chain]]=[
Active Object={
    this:window,
    arguments:{0:"eve",length:1}
    name:"eve"
    func:
    para:"eve"，
},
Global VO={
    this:window,
    app:,
    window: