闭包的应用

    // 应用：封装一段代码
    let xm = (function (){
        let a = 10;
        let b = 20;
        function add(){
            return a + b
        }
        function sub(){
            return a - b
        }
        return {
            add,
            sub
        }
    })()
    let result1 = xm.add()
    let result2 = xm.sub()
    console.log(result1)  // 30
    console.log(result2)  // -10