let const class 类 import 引入模块 export 暴露接口 // var a = 10; // let b = 20; // const c = 30; var和let比 console.log(a) //undefined var a = 10; // 不存在变量提升 console.log(a) //报错 let a = 10; // var a = 10; var a = 20; console.log(a) //// 不允许重复声明 let a = 10; let