babel是javascript编译器，通过设置presets可以将高版本的js转化为低版本的能够在各大没有适配到最新js版本的浏览器上正常运行，实时编译展示工具:https://babeljs.io/repl#?browsers=defaults%2C%20not%20ie%2011%2C%20not%20ie_mob%2011&build=&builtIns=false&spec=false&loose=false&pre_lz=EoUwhgxgLgIg8gWQHQCcQDsAmIUAoA8MAlgGYkAKYA5iAAQD0AfADS2YD2EArgLYZRIaUAKIAbEH3RQAQgE8AkplwAiFO3ZRlASi0BuIA&debug=false&forceAllTransforms=false&shippedProposals=false&circleciRepo=&evaluate=false&fileSize=false&timeTravel=false&sourceType=module&lineWrap=true&presets=env%2Creact%2Cstage-2&prettier=false&targets=&version=7.12.9&externalPlugins=
