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

Start by reading the generic declaration in Fastify's fastify.d.ts and compare its constraint and default syntax with the TypeScript documentation searches listed in the issue. Document what the = http.Server portions mean and add an example or explanation in the appropriate TypeScript documentation; done means newcomers can find and understand this syntax.