defineHostExtension

function defineHostExtension<M extends ExtensionMemberMap>(install: (ctx: HostExtensionContext) => M): HostExtension<M>

Defines a first-class host extension. The installer runs once at construction: it composes via the narrow HostExtensionContext and returns the member map to install. Each returned member is typed onto the host straight from the array — one extension can install MANY typed members.

Opts are supplied by the package’s own factory closure (mirroring ASP.NET services.AddX(opts)), so nothing needs to flow through the framework:

// @flare-ts/drizzle
export function drizzle(opts: DrizzleOptions) {
  return defineHostExtension((host) => {
    host.cfg(DRIZZLE_CONFIG);          // composition runs once, at construction
    host.scoped(DatabaseService);
    return {
      db: makeQueryApi(opts),          // host.db    -- typed
      migrate: () => runMigrations(),  // host.migrate() -- typed
    };
  });
}

// user app -- referencing `drizzle` forces the runtime import; passing it opts in AND types the members.
const host = new FlareHost(node, [drizzle({ url: "..." })]);
host.db.query("select 1");
host.migrate();

Because composition runs once in the installer body (not per member call), passing the extension is the single opt-in: there is no separate activation call.

install (ctx: HostExtensionContext) => M

returns HostExtension<M>

Learn: Host extensions