lexical-binding.js

let obj1 = {
    cb: function () {
        let that = this;
        let fun = function () {
            console.log('this => ', this); // undefined
            console.log('that => ', that); // { cb: [Function] }
        };

        fun();
    }
};

let obj2 = {
    cb: function () {
        let fun = () => {
            console.log('this => ', this); // { cb: [Function] }
        };

        fun();
    }
};