Why cannot use lambda to define prototype function

Question

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.

Answer 1

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);
};

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

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

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

Or better yet, use the new class syntax:

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

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