javascript - vue源码中?:之间无间隔使用是什么意思？

Question

isReservedTag: (x?: string) => boolean;
isReservedAttr: (x?: string) => boolean;
parsePlatformTagName: (x: string) => string;
isUnknownElement: (x?: string) => boolean;
getTagNamespace: (x?: string) => string | void;
mustUseProp: (tag: string, type: ?string, name: string) => boolean;

Answer 1

flow 的类型系统

Answer 2

见这个回答/q/10...