Example: Data model helper

user_helpers.ts

export function isOnVacation(attr: {
  // We pass in the current time, because otherwise this function would rely
  // on the Date global. Instead, the calling component will use a chronometer
  // service which is declaratively passed-in to it.
  nowTime: number;
  // We take advantage of TypeScript's structural typing, so this method can
  // take any compatible User model. This is preferable to relying on a specific
  // User interface: each calling component has a subset of the User object graph
  // that it depends on, so this makes isOnVacation usable in many components.
  user: {
    vacationStartTime(): number;
    vacationEndTime(): number
  };
}): boolean {
  var startTime = attr.user.vacationStartTime();
  var endTime = attr.user.vacationEndTime();

  var wasOnVacation = startTime !== 0 && startTime <= attr.nowTime;
  var stillOnVacation = endTime === 0 || endTime >= attr.nowTime;

  return wasOnVacation && stillOnVacation;
}