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.

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);
}

this doesn't work

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

and this is fine of course:

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

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);
    }
}

Answer 2

The Arrow function would resolve the context this belongs to the scope where the function was defined. I believe you have defined that function in window scope. So the this will point to window in your function.

You can use normal anonymous function here. And we have to be careful while using arrow functions.