How to throw test when property is undefined?

I want this test to throw because it doesn't have a username. Instead, it doesn't throw and shows username as undefined.

Post.js

class Post {

  constructor({username, description, photo}) {
    this.username = username;
    this.description = description;
    this.photo = photo;
  }
}

Post.Test.js

test('post must have a username', () => {
  expect(() => new Post({description: 'a', photo: 'photo.jpg'})).toThrow();
});

How do I make it throw?