条件类型
type IsString = T extends string ? true : false;
type A = IsString<'hello'>; // true
type B = IsString<42>; // false infer 关键字
infer 允许在条件类型中推断类型变量：
type ReturnOf = T extends (...args: any[]) => infer R ? R : never;
type Fn = (x: number) => string;
type R = ReturnOf; // string 模板字面量类型
type EventName = `on${Capitalize}`;
type ClickEvent = EventName<'click'>; // 'onClick'