let count = 10;
    let countObj = {
        count: 1,
        add() {
            function setCount1() {
                console.log(this); //window
                console.log(this.count); //undefined
            }
            let setCount2 = () => {
                console.log(this); //countObj
                console.log(this.count); //1
            }
            setCount1();
            setCount2();
        }
    }

    countObj.add();
    ```