Well, using the type system, weak or strong, to force correct code either at compile time or runtime would help. I can write a function that uses Java reflectio

Well, using the type system, weak or strong, to force correct code either at compile time or runtime would help. I can write a function that uses Java reflection for polymorphism as easily as I can use JavaScript reflection to enforce an API contract. // JavaScript if ( Array.isArray(address) ) { throw new Error("Must only pass a single email address."); } // Java void sendEmail(Object recipientOrRecipients) { ... } There are times when polymorphism is low-risk, and there are times when it's better safe than sorry. Best to know your risk model (and your libraries) and act accordingly.