fix: add generics to Binding type

Signed-off-by: Alex Tennant <atennant@skedulo.com>

Fixes: #487
Signed-off-by: Alex Tennant <atennant@skedulo.com>

Author: adtennant

Diff for: src/message/index.ts

@@ -22,9 +22,9 @@ export * from "./mqtt";
  * @property {@link Deserializer} `toEvent`  - converts a Message into a CloudEvent
  * @property {@link Detector} `isEvent`      - determines if a Message can be converted to a CloudEvent
  */
-export interface Binding {
-  binary: Serializer;
-  structured: Serializer;
+export interface Binding<B extends Message = Message, S extends Message = Message> {
+  binary: Serializer<B>;
+  structured: Serializer<S>;
   toEvent: Deserializer;
   isEvent: Detector;
 }
@@ -65,8 +65,8 @@ export enum Mode {
  * CloudEvent into a Message.
  * @interface
  */
-export interface Serializer {
-  <T>(event: CloudEventV1<T>): Message;
+export interface Serializer<M extends Message> {
+  <T>(event: CloudEventV1<T>): M;
 }
 
 /**

Diff for: src/message/kafka/index.ts

@@ -22,7 +22,7 @@ export type {
  * Bindings for Kafka transport
  * @implements {@linkcode Binding}
  */
- const Kafka: Binding = {
+ const Kafka: Binding<KafkaMessage<unknown>, KafkaMessage<string>> = {
   binary: toBinaryKafkaMessage,
   structured: toStructuredKafkaMessage,
   toEvent: deserializeKafkaMessage,
@@ -35,9 +35,9 @@ type Key = string | Buffer;
  * Extends the base Message type to include
  * Kafka-specific fields
  */
-interface KafkaMessage<T = string> extends Message {
+interface KafkaMessage<T = string | Buffer | unknown> extends Message {
   key: Key
-  value: T | string | Buffer | unknown
+  value: T
   timestamp?: string
 }
 
@@ -61,7 +61,7 @@ interface KafkaEvent<T> extends CloudEventV1<T> {
  * @param {KafkaEvent<T>} event The event to serialize
  * @returns {KafkaMessage<T>} a KafkaMessage instance
  */
-function toBinaryKafkaMessage<T>(event: CloudEventV1<T>): KafkaMessage<T> {
+function toBinaryKafkaMessage<T>(event: CloudEventV1<T>): KafkaMessage<T | undefined> {
   // 3.2.1. Content Type
   // For the binary mode, the header content-type property MUST be mapped directly
   // to the CloudEvents datacontenttype attribute.
@@ -86,7 +86,7 @@ function toBinaryKafkaMessage<T>(event: CloudEventV1<T>): KafkaMessage<T> {
  * @param {CloudEvent<T>} event the CloudEvent to be serialized
  * @returns {KafkaMessage<T>} a KafkaMessage instance
  */
- function toStructuredKafkaMessage<T>(event: CloudEventV1<T>): KafkaMessage<T> {
+ function toStructuredKafkaMessage<T>(event: CloudEventV1<T>): KafkaMessage<string> {
   if ((event instanceof CloudEvent) && event.data_base64) {
     // The event's data is binary - delete it
     event = event.cloneWith({ data: undefined });

Diff for: src/message/mqtt/index.ts

@@ -15,7 +15,7 @@ export type { MQTTMessage };
  * Extends the base {@linkcode Message} interface to include MQTT attributes, some of which
  * are aliases of the {Message} attributes.
  */
-interface MQTTMessage<T> extends Message<T> {
+interface MQTTMessage<T = unknown> extends Message<T> {
   /**
    * Identifies this message as a PUBLISH packet. MQTTMessages created with
    * the `binary` and `structured` Serializers will contain a "Content Type"
@@ -37,7 +37,7 @@ interface MQTTMessage<T> extends Message<T> {
  * Binding for MQTT transport support
  * @implements @linkcode Binding
  */
-const MQTT: Binding = {
+const MQTT: Binding<MQTTMessage, MQTTMessage> = {
   binary,
   structured,
   toEvent: toEvent as Deserializer,