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https://segmentfault.com/q/10...

Answer 3

typescript的语法，表示可选参数和可选属性

function f(x: number, y?: number) {
    return x + (y || 0);
}
f(1, 2);
f(1);
f(1, undefined);
f(1, null); // error, 'null' is not assignable to 'number | undefined'