default custom option value in google-protobuf runtime

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1、I declear one proto 3 file as follows
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 js 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 expected;

Reproduce the behavior with the shown proto3 declaration, generated ext_pb module, and google-protobuf runtime. Start at the getExtension and setExtension entry points, compare their handling of the declared option, and verify that reading the extension returns "Hello world!" before any setter call.