ECMAScript 6 入门
http://es6.ruanyifeng.com/#docs/let
js标准库 js最全的api
https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Array/length
http://es6.ruanyifeng.com/#docs/let
js标准库 js最全的api
https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Array/length