Typescript: message type on SubscriptionCallback<T> based on isRaw

[mrjogo]

Description

createSubscription() takes a callback of type SubscriptionCallback<T>, which is (message: MessageType<T> | Buffer) => void;. However, my understanding is message for a given callback will actually only be one of MessageType<T> or Buffer, depending on the value of the option isRaw passed to createSubscription().

Therefore, instead of having message be the union of MessageType<T> | Buffer, there should be two SubscriptionCallback types (or a generic), one with message: MessageType<T> and one with message: Buffer, and the correct one is used depending on the isRaw option value.

The workaround for the current implementation is to add discriminator checks at the top of all callbacks:

if (Buffer.isBuffer(message)) {
  throw new Error("This should never be a Buffer")
}

• Library Version: 0.27.5
• ROS Version: Jazzy
• Platform / OS: Ubuntu 22.04

Steps To Reproduce

node.createSubscription(
      "std_msgs/msg/String",
      "/my_topic",
      {
        isRaw: false,
      },
      (message) => {
        // Gives error:
        // Type 'Buffer<ArrayBufferLike> | String' is not assignable to type 'String'.
        // Property 'data' is missing in type 'Buffer<ArrayBufferLike>' but required in type 'String'.ts(2322)
        const myString: rclnodejs.std_msgs.msg.String = message;
      }
    );

Expected Behavior

No Typescript error.

Actual Behavior

Typescript error, or discriminator required.