Question

我正在看Static variables in JavaScript，我注意到之前见过的东西，函数已定义，在函数定义之后更新了函数原型：

function MyClass () { // constructor function
  //function definition here
}

//now add a (static?) method *outside* the function definition
MyClass.prototype.publicMethod = function () {
  alert(this.publicVariable);
};

//add a static property *outside* the function definition
MyClass.staticProperty = "baz";

这是我的问题 - 为什么不在函数定义中定义它们，如下所示：

  function MyFunc(){
    MyFunc.staticVar = 1;
    //static method showing static var
    MyFunc.showVarStatic  = function(){
        alert(MyFunc.staticVar);
    }
    //instance method referring to static var
    this.showVarInstance = function(){
        alert(MyFunc.staticVar);
    }
    //instance method - doesn't change static var
    this.inc1 = function(){
        this.staticVar += 1;//no such property
    }
    //static method, changes var
    this.inc2 = function(){
        MyFunc.staticVar += 1;//increments static property
    }
  }

这似乎与IE8，FF和Chrome中的预期一致。这只是个人喜好/风格吗？我喜欢它，因为我的整个函数都包含在那些大括号中。

[编辑：在做了更多的阅读和实验之后，我更好地理解了javascript函数是构造函数，以及它们与C＃类的区别 - 这里是我用来演示的一些代码这]

//this is deceiving, notSoStaticVar won't exist until MyFunc1 has been run
//and then it will be reset whenever MyFunc1 (a constructor) is run
function MyFunc1(){
    MyFunc1.notSoStaticVar = "I belong to MyFunc1";
    this.instanceVar = "I belong to instances of MyFunc1";
}

//this code will be run inline one time, 
//so the static property of MyFunc2 will exist
//(I like how all the functionality is in one code block, but it's kind of messy)
MyFunc2 = (function(){
    var temp = function(){
        this.instanceVar = "I belong to an instance of MyFunc2";
    }
    temp.staticVar = "I belong to MyFunc2";
    return temp;
})();

//this seems to be equivalent to MyFunc2, but the code is cleaner
MyFunc3 = function(){
}
MyFunc3.prototype.instanceVar = "I belong to an instance of MyFunc3";
MyFunc3.staticVar = "I belong to MyFunc3";

//tests
console.log(MyFunc1.notSoStaticVar);//undefined!
var a = new MyFunc1();
console.log(MyFunc1.notSoStaticVar);//"I belong to MyFunc1"
console.log(a.instanceVar);//"I belong to instances of MyFunc1"
MyFunc1.notSoStaticVar = "I will be changed when another instance of MyFunc1 is created";
console.log(MyFunc1.notSoStaticVar);//"I will be changed when another instance of MyFunc1 is created"
var b = new MyFunc1();
console.log(MyFunc1.notSoStaticVar);//"I belong to MyFunc1" - was reset by constructor!

//now test MyFunc2
console.log(MyFunc2.staticVar);//"I belong to MyFunc2"
MyFunc2.staticVar = "I am not affected by the construction of new MyFunc2 objects";
var c = new MyFunc2();
console.log(c.instanceVar);//"I belong to an instance of MyFunc2"
console.log(MyFunc2.staticVar);//"I am not affected by the construction of new MyFunc2 objects"

//now test MyFunc3
console.log(MyFunc3.staticVar);//"I belong to MyFunc3"
MyFunc3.staticVar = "I am not affected by the construction of new MyFunc3 objects";
var d = new MyFunc3();
console.log(d.instanceVar);//"I belong to an instance of MyFunc3"
console.log(MyFunc3.staticVar);//"I am not affected by the construction of new MyFunc3 objects"

//interesting
console.log(c);//"temp" <-- not really intuitive!
console.log(d);//"MyFunc3" <-- makes sense

Answer 1

简而言之：性能。

在函数内部定义它们将导致每次调用构造函数时都重新定义它们。

虽然这会像预期的那样表现，但没有充分的理由只是开销。

Answer 2

因为它会为每个对象实例添加唯一的函数。这会消耗额外的内存开销，而这通常是不必要的。

在某些情况下它可能很有用，比如函数应该引用局部变量，但如果情况不是这样，那么它们应该在原型上。

此外，静态的将不断被覆盖。

为什么在函数定义之外引用静态变量（函数属性）？

2 个答案: