docs: better subscription error handling [skip ci]

Author: enisdenjo

README.md

@@ -369,24 +369,20 @@ function fetchOrSubscribe(operation: RequestParameters, variables: Variables) {
       {
         ...sink,
         error: (err) => {
-          if (err instanceof Error) {
-            return sink.error(err);
-          }
+          if (Array.isArray(err))
+            // GraphQLError[]
+            return sink.error(
+              new Error(err.map(({ message }) => message).join(', ')),
+            );
 
-          if (err instanceof CloseEvent) {
+          if (err instanceof CloseEvent)
             return sink.error(
-              // reason will be available on clean closes
               new Error(
-                `Socket closed with event ${err.code} ${err.reason || ''}`,
+                `Socket closed with event ${err.code} ${err.reason || ''}`, // reason will be available on clean closes only
               ),
             );
-          }
 
-          return sink.error(
-            new Error(
-              (err as GraphQLError[]).map(({ message }) => message).join(', '),
-            ),
-          );
+          return sink.error(err);
         },
       },
     );
@@ -460,26 +456,20 @@ class WebSocketLink extends ApolloLink {
           next: sink.next.bind(sink),
           complete: sink.complete.bind(sink),
           error: (err) => {
-            if (err instanceof Error) {
-              return sink.error(err);
-            }
+            if (Array.isArray(err))
+              // GraphQLError[]
+              return sink.error(
+                new Error(err.map(({ message }) => message).join(', ')),
+              );
 
-            if (err instanceof CloseEvent) {
+            if (err instanceof CloseEvent)
               return sink.error(
-                // reason will be available on clean closes
                 new Error(
-                  `Socket closed with event ${err.code} ${err.reason || ''}`,
+                  `Socket closed with event ${err.code} ${err.reason || ''}`, // reason will be available on clean closes only
                 ),
               );
-            }
 
-            return sink.error(
-              new Error(
-                (err as GraphQLError[])
-                  .map(({ message }) => message)
-                  .join(', '),
-              ),
-            );
+            return sink.error(err);
           },
         },
       );