defineProperty 描述符参数和 Object.prototype 的困境

Question

我正在使用defineProperty创建对象属性，函数中的descriptor参数是否一定需要是object。代码如下：

var config=new Object();
var defineProp = function ( obj, key, value ){
config.value = value; // why should this parameter be an object?
Object.defineProperty( obj, key, config );
};

为什么一定要给Descriptor参数传递一个Object

我有以下两个代码片段，我在其中创建一个构造函数，然后使用它创建对象。这两个代码在控制台中返回相同的输出。使用.prototype.Methodname 会改变什么吗？

1

function Car( model, year, miles ) {
this.model = model;
this.year = year;
this.miles = miles;
}

Car.prototype.toString = function () {
return this.model + " has done " + this.miles + " miles";
};
// Usage:
var civic = new Car( "Honda Civic", 2017, 30000 );

console.log( civic.toString() );

2

function Car( model, year, miles ) {
this.model = model;
this.year = year;
this.miles = miles;
this.toString = function () {
return this.model + " has done " + this.miles + " miles";
};
}

var civic = new Car( "Honda Civic", 2017, 30000 );
console.log( civic.toString() );

代码用法如下：

var civicSport= Object.create( person );

defineProp(civicSport, "topSpeed", "120mph");//function created above
console.log(civicSport);

Answer 1

descriptor参数指的是正在定义或修改的属性，所以我们需要将其称为键值对（key=正在定义/修改的属性，值= 修改或赋值给属性）。因此descriptor 必须是Object

更多详情请见MDN docs