Undocumented = operator syntax in Generic Constraining

In the declaration file for Fastify (https://github.com/fastify/fastify/blob/master/fastify.d.ts) there is some TypeScript code I do not recognize and cannot find another example of in the TypeScript documentation:

declare function fastify<
  HttpServer extends (http.Server | http2.Http2Server) = http.Server,
  HttpRequest extends (http.IncomingMessage | http2.Http2ServerRequest) = http.IncomingMessage,
  HttpResponse extends (http.ServerResponse | http2.Http2ServerResponse) = http.ServerResponse
>(opts?: fastify.ServerOptions): fastify.FastifyInstance<HttpServer, HttpRequest, HttpResponse>;

I recognize the Generic constraint syntax with the extends keyword, but what about the = http.Server part? Is this like a default parameter or something?

Whatever this syntax means it should be better documented.

I tried googling for this syntax by searching

• typescript generic constraint
• typescript generic constraint assignment
• typescript generic defaults

首先阅读 Fastify 的 fastify.d.ts 中的泛型声明，并将其约束和默认值语法与 issue 中列出的 TypeScript 文档搜索结果进行比较。记录 = http.Server 部分的含义，并在相应的 TypeScript 文档中添加示例或说明；完成标准是新手能够找到并理解这一语法。