Question

在下面的代码中，我无法将b实例化为新的a（），但能够将其作为新的f（）进行实例化。我的结论是新的工作，对象必须是函数的直接/直接后代。那是准确的吗？我对javascript相对较新，并且正努力从根本上理解该语言。感谢，

＆＃13;

var f = new Function();
console.log("Prototype of f:" + f.prototype);
console.log("Constructor of f:" + f.constructor);
console.log("Prototype Link of f:" + f.__proto__);
if (f instanceof Function) console.log("f isa Function");


function A() {}
console.log("Prototype of A:" + A.prototype);
console.log("Constructor of A:" + A.constructor);
console.log("Prototype Link of A:" + A.__proto__);

var a = new A();
console.log("Prototype of a:" + a.prototype);
console.log("Constructor of a:" + a.constructor);
console.log("Prototype Link of a:" + a.__proto__);
if (a instanceof Function) console.log("a isa Function");
if (a instanceof A) console.log("a isa A");


var b = new f();
console.log("Prototype of b:" + b.prototype);
console.log("Constructor of b:" + b.constructor);
console.log("Prototype Link of b:" + b.__proto__);

 //  b = new a();
 // error: Uncaught TypeError: object is not a function (anonymous function)

＆＃13;

Answer 1

看看那个......

var f = new Function();
console.log("Prototype of f:" + f.prototype);
console.log("Constructor of f:" + f.constructor);
console.log("Prototype Link of f:" + f.__proto__);
if (f instanceof Function) console.log("f instanceof Function");
if (f instanceof Object) console.log("f instanceof Object");

function A() {}
console.log("Prototype of A:" + A.prototype);
console.log("Constructor of A:" + A.constructor);
console.log("Prototype Link of A:" + A.__proto__);
if (A instanceof Function) console.log("A instanceof Function");
if (A instanceof Object) console.log("A instanceof Object");

var a = new A();
console.log("Prototype of a:" + a.prototype);
console.log("Constructor of a:" + a.constructor);
console.log("Prototype Link of a:" + a.__proto__);
if (a instanceof A) console.log("a instanceof A");
if (a instanceof Object) console.log("a instanceof Object");

Answer 2

Function constructor创建一个新的Function对象。在JavaScript中每个函数实际上都是一个Function对象。

var f = new Function();//f is a function and not an normal object
var b = new f();//work expected as f is a function

新a（）不起作用的原因

function A() {}//A is function
var a = new A();//a is not a function rather an object
var b = new a();//throw error as 'new' expect function

<强> new operator: 新运算符需要constructor function。

var a = new foo[([arguments])]//foo must be a function.

当执行代码new foo（...）时，会发生以下情况：

创建一个新对象，继承自foo.prototype。
使用指定的参数调用构造函数foo，并将this绑定到新创建的对象。 new foo等同于新的foo（），即如果没有指定参数列表，则不带参数调用foo。

构造函数返回的对象成为整个新表达式的结果。如果构造函数没有显式返回对象，则使用在步骤1中创建的对象。（通常构造函数不会返回值，但如果他们想要覆盖正常的对象创建过程，他们可以选择这样做。） e.g

function f() {
    this.y = 50;//will be overridden due to below return statement
    return function () {
        this.x = 5;
    }
}
var b = new f();//b is a function as f() is returning a function and hence it will override the normal function creation
alert(b.y);//undefined as y is no more property of instance b as function constructer has a return statement returning object(function object in this case)
var c = new b(); 
alert(c.x);//c.x = 5;

//错误：未捕获TypeError：对象不是函数（匿名函数）

2 个答案: