为什么不能使用lambda来定义原型函数 [英] Why cannot use lambda to define prototype function

问题描述

有人可以解释为什么用lambda表达式定义原型函数不起作用？我以为必须先问这个但是找不到它。

Can someone please explain why defining a prototype function with lambda expression doesn't work? I thought this must be asked before but couldn't find it.

function Book(title, year) {
    this.title = title;
    this.year = year;

    // define a function within the object, which works fine
    this.printYear = () => console.log("instance function of an object: " + this.year);
}

这不起作用

Book.prototype.printTitle2 = () => {
        console.log(this.title);
    }

当然没问题：

Book.prototype.printTitle = function() {
         console.log(this);
         console.log(this.title);
    }

推荐答案

其中一个主要特征箭头函数是它们从它们创建的上下文中关闭 这个;他们根据他们像其他功能一样被调用的方式得不到它。所以......

One of the chief features of arrow functions is that they close over the this from the context in which they're created; they don't get it based on how they're called like other functions do. So...

// ...whatever `this` is *here*
Book.prototype.printTitle2 = () => {
    // ...is what `this` will be *here*
    console.log(this.title);
};

但是你的函数依赖于这个会有所不同关于如何调用它。

But your function relies on this varying depending on how it's called.

这不是箭头函数的用例。使用普通函数：

This just isn't a use-case for arrow functions. Use a normal function:

Book.prototype.printTitle2 = function() {
    console.log(this.title);
};

或者更好的是，使用新的类语法：

Or better yet, use the new class syntax:

class Book {
    constructor(title, year) {
        this.title = title;
        this.year = year;
    }

   printTitle2() {
        console.log(this.title);
    }
}

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