javascript - What does it mean to use ?: without interval in vue source code?

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’s type system

Answer 2

See this answer/q/10...