When I executed the toObject method on a message with a field marked optional, that field became the default value like 0 or "". I expect it would be undefined.

I guess it is because the third argument of getFieldWithDefault is not undefined.
Since proto3 currently support optional keyword, when optional is specified, I think it should be undefined if it is not given.

proto

syntax = "proto3";

package com.book;

message OptTest {
    optional string name = 1;
}

generated toObject code

proto.com.book.OptTest.toObject = function(includeInstance, msg) {
  var f, obj = {
    name: jspb.Message.getFieldWithDefault(msg, 1, "")
  };

  if (includeInstance) {
    obj.$jspbMessageInstance = msg;
  }
  return obj;
};

versions

$ npm list --depth=0 -g
/usr/local/lib
+-- google-protobuf@3.21.2
+-- grpc_tools_node_protoc_ts@5.3.3
`-- grpc-tools@1.12.4