loadsh set new key to empty interface Object , how to prevent it or improve on initialization?

Hi I am trying to declare a generic interface with multiple elements and try to assign in object keys values from other JSON objects but need to assign particular key from the interface , here is the detail code

interface DbInterface {
  userName: string;
  password: string;
port: string;
}

auth = {
"userName" :"admin",
"password" : "password"
"previousPassword" : "admin"
}

let dbConfig: DbInterface = {}
for (const key in auth) {

lodash.set(dbConfig, key, value)
}

It’s Returning

dbConfig: {
  "userName": "admin";
  "password": "password";
"previousPassword":"admin";
}

instead of

dbConfig: {
  "userName": "admin",
  "password": "password"
}

What is my issue?