docs: Explain the typecasts in provider-from-middleware

rekmarks · rekmarks · commit 4a63eeab480f · 2025-11-05T11:08:48.000-08:00
diff --git a/packages/eth-json-rpc-provider/src/provider-from-middleware.ts b/packages/eth-json-rpc-provider/src/provider-from-middleware.ts
@@ -24,6 +24,10 @@ export function providerFromMiddleware<
   Result extends Json,
 >(middleware: LegacyJsonRpcMiddleware<Params, Result>): InternalProvider {
   return providerFromMiddlewareV2(
+    // This function is generic on the Params and Result types to match the legacy JsonRpcMiddleware type.
+    // However, since the V2 JsonRpcMiddleware type is not generic on the Params, we need to elide this
+    // parameter by upcasting the request type to JsonRpcRequest, or we get an error due to contravariance
+    // since JsonRpcRequest<Params> is not assignable to JsonRpcRequest.
     asV2Middleware(middleware) as JsonRpcMiddleware<JsonRpcRequest>,
   );
 }
@@ -44,6 +48,9 @@ export function providerFromMiddlewareV2<
 >(middleware: Middleware): InternalProvider {
   return new InternalProvider({
     engine: JsonRpcEngineV2.create({
+      // This function is generic in order to accept middleware functions with narrower types than
+      // the plain JsonRpcMiddleware<JsonRpcRequest> type. However, since InternalProvider is non-generic,
+      // we need to upcast the middleware to avoid a type error.
       middleware: [middleware as JsonRpcMiddleware<JsonRpcRequest>],
     }),
   });
