JavaScript 将原型(prototype)添加到现有对象

Question

function abc(){
 this.a = "Hey this is A";   
}

var va = new abc();

va.prototype = function(){
  this.b = 'Hey b is added';
}

console.log(va.b);

va.b 未定义，我以为我已经将它添加到 va 了？因为 console.log(va) 是一个对象，如果我在原型(prototype)行之前进行控制台。我上面的代码有什么问题？

Answer 1

看起来你真的只是想给 va 添加一个属性:

va.b = "Hey b is added";

如果，但是，您想要扩充 va 已有的原型(prototype)，您可以通过引用 va 的原型(prototype)来实现对象，您可以通过多种方式获得:

• 根据上面的代码，通过 abc.prototype

• 或者在 ES5+ 浏览器上，通过 Object.getPrototypeOf(va)

例如:

function Abc() {
  this.a = "Hey this is A";
}

var va = new Abc();
snippet.log(va.a); // "Hey this is A"
snippet.log(va.b); // undefined

Abc.prototype.b = 'Hey b is added';

snippet.log(va.b); // "Hey b is added"

<!-- Script provides the `snippet` object, see http://meta.stackexchange.com/a/242144/134069 -->
<script src="http://tjcrowder.github.io/simple-snippets-console/snippet.js"></script>

请注意，向原型(prototype)添加属性意味着所有使用该原型(prototype)的对象都将继承该属性:

function Abc() {
  this.a = "Hey this is A";
}

var a1 = new Abc();
var a2 = new Abc();

Abc.prototype.b = 'Hey b is added';

snippet.log(a1.b); // "Hey b is added"
snippet.log(a2.b); // "Hey b is added"

<!-- Script provides the `snippet` object, see http://meta.stackexchange.com/a/242144/134069 -->
<script src="http://tjcrowder.github.io/simple-snippets-console/snippet.js"></script>