arrow-explanation.js

const Foo = function () {}
Foo.prototype.bar = "bar",
Foo.prototype.arrow = () => {
    console.log(this.bar);
};
Foo.prototype.fn = function () {
    console.log(this.bar);
};
Foo.prototype.handyArrow = function () {
    process.nextTick(() => console.log(this.bar));
};
Foo.prototype.badFn = function () {
    process.nextTick(function () {
        console.log(this ? this.bar : undefined);
    });
};

const f = new Foo;
f.arrow();
f.fn();
f.handyArrow();
f.badFn();