Try this way:

const { firstUser } = obj;
const { name, age } = firstUser;
const { lastName: Lname } = name; // this is destructuring assignment

Now you can use lastName as Lname variable.

Because you have placed the age field inside firstName object,nested inside the obj. I have refactored your code

 const obj = {
  firstUser: {
    name: {
      firstName: 'John',
      lastName: 'Doe',
    },
  },
  age: 32,
}


const {firstUser:{name: {lastName: Lname}}, age} = obj;

console.log(Lname);
console.log(age);