Typescript泛型未提供时，如何设置ICreateItem的optionalFields默认类型？

需求说明：

• 未指定ICreateItem泛型参数时，optionalFields属性类型为OptionalFieldsByTopic<Topic>
• 指定泛型参数T时，optionalFields属性类型为OptionalFieldsByTopic<T>

原代码中泛型默认值设为never导致未传泛型时类型不符合预期，只需将泛型的默认参数改为Topic即可实现需求：

// 修改泛型默认值为Topic，实现默认类型适配
export interface ICreateItem<T extends Topic = Topic> {
  id: string;
  name: string;
  tags: string[];
  collectionId?: string;
  topic: string;
  optionalFields?: OptionalFieldsByTopic<T>;
}

type OptionalFieldsByTopic<T extends keyof IOptionalFields> = IOptionalFields[T];

export type Topic = keyof IOptionalFields;

export interface IOptionalFields {
  books: {
    author?: string;
    language?: string;
    translation?: string; 
  };
  vehicle: {
    model?: string;
    type?: string;
    color?: string;
  };
  painting: {
    author?: string;
    description?: string;
    image?: string;
  };
}

效果验证 #

• 不指定泛型时：

  // optionalFields 类型为 books | vehicle | painting 的联合类型
  const item: ICreateItem = {
    id: "1",
    name: "测试项",
    tags: ["test"],
    topic: "books",
    optionalFields: { author: "brandon", language: "english" } // 符合类型要求
  };
  

• 指定泛型时：

  // optionalFields 类型严格匹配books对应的结构
  const bookItem: ICreateItem<"books"> = {
    id: "2",
    name: "某本书",
    tags: ["literature"],
    topic: "books",
    optionalFields: { author: "J.K.Rowling", translation: "中文" } // 符合books类型约束
  };
  

内容的提问来源于stack exchange，提问作者Lukasz