custom option of protocol buffer 3 in javascript

1、I declear one proto 3 file

syntax = "proto3";

import "google/protobuf/descriptor.proto";

extend google.protobuf.MessageOptions {
  optional string my_option = 51234;
}

message Ext {
  option (my_option) = "Hello world!";
}

2、And then I compile it, and use in google-protobuf runtime

import { Ext, myOption } from './ext_pb'
const ext = new Ext()

3、 And then, if I execute the following code

console.log(ext.getExtension(myOption))

I think it would output “Hello world!”, but it ouput “undefined”, why this happen;

4、If I execute the following code

ext.setExtension(myOption, 'se')
console.log(ext.getExtension(myOption))

It would output ‘se’ as my expected;