Typescript property reference inside a property [duplicate]

I have a property declared below:

export const properties = {
  username: 'admin',
  password: 'pass',
  environment: 'test1',
  // Trying to construct the url so it becomes http://test1.com
  url: 'http://'  + environment + '.com',

};

I would like to reuse environment inside the url value. However it can’t seem the find it. I have tried the following and had no luck:

url: 'http://'  + environment + '.com',
url: 'http://'  + this.environment + '.com',
url: 'http://'  + properties.environment + '.com',

Does anyone have any idea on how it can be done? Thank you