javascript - 使用 JS 多态性和方法重写

Question

我目前正在学习 javascript 的基础知识，但我很难理解这个图(如下)。调用 FirmAnswer.get() 时，为什么值是 42!!？不是应该是未定义的吗？因为如果我没有记错的话，function fn2() 中 return this.val 中的 this 指的是firmAnswer。

归功于图表所有者

Answer 1

because if I'm not mistaken the this in return this.val in function fn2() refers to firmAnswer

是的，确实如此，但是当您尝试从对象检索属性 (val) 时，如果对象本身没有该属性，JavaScript 引擎将查找该对象的原型(prototype)。 firmAnswer 的原型(prototype)是 answer(因为这是 Object.create 用于设置的)，确实有一个 val，因此使用该属性的值。这称为原型(prototype)链，是原型(prototype)继承的关键。

这是一个更简单的示例:

// Create an object with a `val`
var p = {
  val: "p's value"
};

// Create an object using `p` as its prototype
var o = Object.create(p);

// Use `val` from `o` -- since `o` doesn't have `val`, the engine
// looks to `p`
snippet.log(o.val); // "p's value"

// Give `o` its own `val`
o.val = "o's value";

// Use it
snippet.log(o.val); // "o's value"

// Since this is based on whether `o` has `val` at all
// (not the *value* of `val`), we can *remove* `val`
// from `o` and start using `p`'s again:

// Remove `val` from `o` entirely
delete o.val;

// Use `val` again -- and we're back to using `p`'s
snippet.log(o.val); // "p' s value ""

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