有一个问题,我不明白,任何人都可以理解它吗？

Question

来自在线资源文件的问题:

Create javascript so that the following methods 
produce the output listed next to them.

    circle = new Circle();
    console.log(circle.get_area());  // 3.141592653589793
    circle.set_radius(10);
    console.log(circle.get_area());  // 314.1592653589793
    console.log(circle);             // the radius of my circle is 10 and its area is 314.1592653589793

谁能理解被问到的是什么？

以下是我的评论的副本,格式相同:

function Circle() {
    this.pi=3.141592653589793;
    this.radius;
}  

Circle.prototype={
    get_area: function(){
        return this.radius*this.pi;
    },
    set_radius: function(radius){  
        return this.radius=radius;
    }  
};  

circle=new Circle();
circle.set_radius(100);

好吧,我已经玩过这个并且有点了解正在发生的事情,尽管我不确定何时或为什么需要用这种技术写作; 如果有人可以解释我可能会更好地了解它的用途.

我的最终代码如下: -

function Circle(r) {
    this.pi=Math.PI;
}  

Circle.prototype={
    get_area: function(){
        return this.radius*this.pi;
    },
    set_radius: function(radius){  
        return this.radius=radius;
    }
};  

var circle=new Circle();
circle.set_radius(100);

Circle.prototype.toString=function(){
    return 'The radius of my circle is '+circle.radius+' and it\'s area is '+this.get_area();
}

console.log(circle);

我不完全确定我是否正确使用Circle.prototype.toString = function(),因为它似乎只是创建一个字符串.

Answer 1

他们要求你创建一个构造函数叫做Circle它创建,将有对象get_area,set_radius以及toString在指定的行为方式的功能.(toString将在最终语句中使用,您将输出circle实例的值.)他们可能希望您通过Circle原型为对象提供这些功能(见下文).

Circle是构造函数,因为它创建了新对象(从技术上讲,它只是填充它们;它们是由运算符创建的new).有时人们会调用这些类,但它们并不是真正基于类的OOP方式的类(它们是JavaScript使用的基于原型的OOP方式的构造函数).

有几种方法可以做到这一点,但同样,通常你会有一个Circle函数来设置实例的属性this,然后将函数分配给Circle.prototype,通过它们创建的所有对象new Circle都将继承.我举了一个例子,但我的印象是这是一个学习练习,所以最好留给读者.但有一些指示:

• 在构造函数内部,您可以new使用关键字引用运算符创建的对象this.
• 构造函数对象将在其上调用一个属性prototype.如果为该原型分配属性,它们将由使用构造函数创建的实例继承(通过称为原型链的东西继承).因此,如果我有一个构造函数Foo,我可以bar在Foo.prototype(Foo.prototype.bar = /* whatever */;)上设置一个属性(比如说),并且通过创建的所有实例new Foo()都将具有一个bar具有该值的属性.
• JavaScript中的函数是一流的对象.您可以像引用字符串或数字一样引用它们.如果我有function foo() { /* ... */ },我可以设置一个变量x来引用它(var x = foo;)然后通过该变量(x();)调用它.
• 就像x可以引用一个函数一样,bar我上面提到的属性可以引用一个函数.

希望这能让你走上正确的轨道,而不必完全放弃游戏.

重新评论有关JavaScript的更多信息的参考文献:

• 我发现David Flanagan(来自O'Reilly)的书" JavaScript:The Definitive Guide"非常好.它可能会有点过时,第五版现在已有几年了.
• 在http://javascript.crockford.com上有很多来自备受尊敬的JavaScript大师Douglas Crockford的文章,但我警告你,Crockford很简洁.:-)聪明而明智,即使一个人并不总是同意他的结论.
• 我真的不喜欢把我的博客[因为A),所以不是自我推销,和B)"贫血"不半说]但也有一对夫妇的职位有,我想可能有帮助.

我的评论如下:真的我不能没有一个例子,因为这样做没有帮助.所以这是构造函数和与之关联的一些函数的示例:

// A constructor function called `Person`
function Person(fname, lname) {

    // Within the constructor, we can refer to the new instance via `this`.
    // Let's remember the names we were given.
    this.firstName = fname;
    this.lastName = lname;

    // Now, the instance that is returned from `new Person(...)` will have
    // those properties on it.
}

// Let's give the Person prototype a function that returns the person's
// full name.
Person.prototype.getFullName = function() {

    return this.firstName + " " + this.lastName;
};

// Okay, let's see `Person` in action:
var p = new Person("John", "Doe");
console.log(p.firstName);     // "John" -- this is referencing the property we set in the constructor
console.log(p.getFullName()); // "John Doe" -- this calls the function we get from the prototype

// `p` has the `getFullName` function because it inherits it from the
// `Person.prototype` object.

// Let's see what happens if I try to output the instance itself:
console.log(p); // Most like you get "[object Object]" or similar -- not useful!

// In that sort of situation, JavaScript will use a function called `toString`
// to get a string equivalent of the instance. (It's more complicated than that,
// but you don't want me to go into the details here.) So let's give
// `Person.prototype` a `toString` function that does something useful:
Person.prototype.toString = function() {

    return this.getFullName();
};
// Note there that we're using `this` again. When a function is called through
// an object property (e.g., `p.toString()`), within the function call `this`
// refers to the object instance. `this` is an important and flexible concept
// in JavaScript and very much unlike its counterpart in (say) Java or C++,
// even though in simple situations (like this) it looks similar.

// Now let's try it:
console.log(p); // "John Doe", because the interpreter implicitly calls `toString` for you

// "Hey, wait a minute!" I hear you saying, "You didn't make a new `p`! How did it
// get the new function?!" The answer is it got it the same way it got `getFullName`:
// from `Person.prototype`. The `p` instance *refers* to the `Person.prototype`, it
// doesn't have a *copy* of it. So changes to `Person.prototype` show up in `p`
// (unless `p` overrides them, which we aren't doing in this example).

(那个例子使用匿名函数,我不喜欢它,但我不想在这里讨论命名函数的整个讨论.)

(OT:挑剔,向你提出这个问题的人真的应该circle在最开始时声明变量:var circle;另外,请注意,console.log在所有JavaScript实现中可能都不存在 - 它最好从Firefox + Firebug中获知.)