data.js

userPlan = {
  // plenty of stuff
  startingWeek = Date object; // such as Mon Feb 19 2018 16:34:08 GMT+0100 (W. Europe Stand…
  // other stuff
}

function saveData(data){
  console.log(this) // => correctly logs the userPlan object, including startingWeek property
  console.log(this.data) // logs 'undefined'
  localStorage.setItem(data, JSON.stringify(this.data)); // => saves: 'startingWeek' : undefined;
}

saveData.call(userPlan, 'startingWeek');