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  • 第7章函数表达式笔记

    第 7章 函数表达式

    .声明函数

    1.使用关键字 function

    Function functionname(参数1,参数2){}

    此方法的特征是:函数声明提升,意思就是调用可以放在声明的前面

    2.使用函数表达式

    Var functionname=function(参数1,参数2{}

    此种方法更像是常量之间的赋值语句,即创建一个函数并将它赋值给一个变量,此种创建的 函数称为匿名函数,因为function关键字后面没有标识符。

    区别:

    函数声明:

    If(condition){

    Function sayHi(){

    Alert(“hi”)

    }

    }

    Else{

    Function sayHi(){

    Alert(“Yo”)

       }

    }

    词语法是无效的ECMAscript语法,

    但是可以函数表达式:

    Var sayHi;

    If(condition){

    sayHi=function(){

    Alert(“hi”)

    }

    }

    Else{

    sayHi=function(){

    Alert(“Yo”)

       }

    }

    二. 递归

    函数递归就是一个函数通过名字调用自身的情况下构成的,

    因此可以用的,arguments.callee属性

    1.函数内部的属性

    a)  Arguments:他是一个类数组对象,包含着传入函数中的所有参数,它还有一个名为 callee的属性,该属性是一个指针,指向拥有Arguments对象的函数(当前运行的函数)

    类如计算阶乘:

      Function factorial(num){

     If(num<1){

    Return 1

    }else{

    Return num*factorial.callee(num-1)

           }

      b)  this 引用的是函数据以执行的环境对象。

      c)  caller这个属性保存着调用当前函数的函数的引用,如果是在全局作用域中调用当前 函数,它的值为null.

    三. 闭包

    闭包就是有权访问另一个函数作用域中的变量的函数。创建一个闭包最常见的方法就是一个函数内部创建另一个函数(然后把被包含的函数作为变量返回)

    function f1(){

        n=999;

        function f2(){

          alert(n);

        }

        return f2;

      }

      var result=f1();

      result();  因为发f1返回的是f2函数,所以在result()调用时,类似调用f2函数

    1.闭包与变量

    由于作用域链的这种配置机制引出了一个值得注意的副作用,即闭包只能取得包含函数中任何变量的最后一个值.别忘了闭包所保存的是整个变量对象,而不是某个特殊的对象.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         

    :function createFunctions(){

    Var result=new Array();

    For(var i=0;i<10;i++){

    Result[i]=function(){

    Return i;

    }

    }

    Return result}

       以上代码看似返回的是每个函数的索引,但是实际每个函数返回的都是10;要想实现每个函数返回的都是10,则加另一个匿名函数

    function createFunctions(){

    Var result=new Array();

    For(var i=0;i<10;i++){

    Result[i]=function(num){

    Return function(){

    Return num;

    };

    }(i);

    }

    Return result}

    2.关于this对象

    因为匿名函数的执行环境具有安全性,因此其this对象通常指向window.

      如:

    Var name=”the window”

    Var object={

    Name:”my object”;

    getNameFunc:function(){

    Return function(){

    Return this.name

    }

    }

    }

    Alert(object.getNameFunc()());   //the window

    可以把外部作用域中的this对象保存在一个闭包能够访问到的变量里就可以让闭包访问该对象了;

    Var name=”the window”

    Var object={

    Name:”my object”;

    getNameFunc:function(){

    Var that=this

    Return function(){

    Return that.name

    }

    }

    }

    Alert(object.getNameFunc()());   //my obejct

    3.内存泄露

    如果闭包作用域中保存一个html元素,那么就意味着该元素无法被销毁;

    Function assignhandler(){

    Var element=documentById(“someElement”);

    Element.onclick=function(){

    Alert(element.id)

    };

    }

    以上代码创建一个作为element元素事件处理程序的闭包,而这个闭包又创建了一个循环引用,由于匿名函数保存一个对assignhandler()的活动对象的引用。因此就导致无法减少对element的引用数,因此他占用内存就永远不会被收回。

    改进:

    Function assignhandler(){

    Var element=documentById(“someElement”);

    Var id=element.id;

    Element.onclick=function(){

    Alert(id)

    };

    Element=null;

    }

    通过把 elemenyt.id的一个副本保存在一个变量里,而且在闭包中引用该变量,消除了循环引用。闭包会引用包含函数的整个活动对象,其中包含着element,即使闭包不直接引用element,包含函数的活动对象中也仍然会保存一个引用。因此有必要把element设为Null.    (包含函数就是父函数)

    4.模仿块级作用域

    JavaScript中没有块级作用域的概念,意味着在块语句定义的变量,实际上是在包含函数中而非语句中创建的,

    把函数声明转换为函数表达式,只要像下面:

    (function (){

    /这里是块级代码作用域

    ])()

    像上面一样如需零时的需要一些变量就可以使用私有作用域

    Function outputNumbers(count){

    (function () {

    For(var i=0;i<count;i++){

    Alert(i)

    }

    })();

    Alert(i)  //导致一个错误

    }以上代码我们在for循环外部插入了一个私有作用域,在匿名函数中定义的任何变量,都会在执行结束时被销毁,因此,变量I只能在循环中使用。

    5.私有变量

    任何在函数中定义的变量都是私有变量,因为不能在函数外部访问这些变量

    1.私有变量包括:函数的参数 局部变量 函数内部定义的其他函数,

    2.通过闭包可以访问私有变量,利用这一点就可以创建用于访问私有变量的公有方法。我们把有权访问私有变量和私有函数的公有方法称为特权方法。

    Function myObject(){

    //私有变量和私有函数

    Var privateVariable=10;

    Function privateFunction(){

    Return fasle;

    }

    //特权方法

    this.publicMethod=function(){

    privateVariable++;

    Return privateFunction();

    }

    }

    对与以上而言,privateVariableprivateFunction()只能通过特权方法来访问。在创建myObject实例后,只有这一个方法才可以访问以上私有变量和私有函数

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  • 原文地址:https://www.cnblogs.com/danson/p/4501200.html
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