How does mobile safari determine when to prompt the user to share location in HTML?

When we have a requirement in which we want to track the latest locations for a user only when they are in a certain area, then we write separate code for it. The code to determine when to prompt the user to share location in HTML is as follows -

if (frstTime) { //First time    
   navigator.getCurrentPosition(function (coordinates) {      
      if (coordsAreInTheBox) {          
         storeCoordsForUser();          
         navigator.watchPosition();      
      }    
   });
}  

else if (userInlocat) {    
   navigator.watchPosition();
}

It works as expected on the first load. It will check for whether the user came for the first time; if yes, then the navigator gets the current position. After that, according to coordinates in the box, we watch positions. If the user is in location then watch position.