Here, we have a function called schedulerMaker, which has a single parameter that expects a name string. This function then returns another function, which accepts an event as its parameter. Finally, this function itself returns a final function, which when called, logs a string describing our event. That’s a lot of functions, so let’s try to follow what is happening.

First, we call schedulerMaker with the argument “Ada” and it gives us a new function, which we assign to the variable adaScheduler. For its part, adaScheduler closes over the variable name, which contains the value of “Ada”, and therefore retains permanent access to it. We then call adaScheduler with the argument “Coffee”, which it assigns to the variable event, and it in turn returns another function that is going to describe our event. Now, we have a function called coffeeWithAda that has, in effect, closed over two levels of scope, thus giving it access to both the event and name variables. And indeed, when we call coffeeWithAda, the message “Coffee with Ada” is logged to the console.

Keep in mind that the event and name variables we are using here are not universal. If we wanted, we could use schedulerMaker to create many more functions, each with a different name attached. We could then in turn use those functions to create many more event describers, and each one would have access to different values.

Private Data and Application Interfaces

Two of the biggest benefits of closure are the ability to create private data and to define application interfaces. Sometimes, you want to enforce the way in which a program interacts with data so that you can protect its integrity. By using closure, you can do exactly this. One common way of creating such an interface is by returning an object from a function. Methods defined on this object, just like any other functions, close over the current scope. This means that we can define private data inside the overall function scope, which will only be accessible to methods defined on our object. Here is an example:

function makeCalendar(name) {
  var calendar = {
    owner: name,
    events: [],
  };
  
  return {
    addEvent: function(event, dateString) {
      var eventInfo = {
        event: event,
        date: new Date(dateString),
      };
      calendar.events.push(eventInfo);
      calendar.events.sort(function(a, b) {
        return a.date - b.date;
      });
    },
    
    listEvents: function() {
      if (calendar.events.length > 0) {
        console.log(calendar.owner + "'s events are: ");
        
        calendar.events.forEach(function(eventInfo) {
          var dateStr = eventInfo.date.toLocaleDateString();
          var description = dateStr + ": " + eventInfo.event;
          
          console.log(description);
        });
      } else {
        console.log(calendar.owner + " has no events.");
      }
    },
  };
}

var babbageCalendar = makeCalendar("Charles Babbage");

babbageCalendar.addEvent("Coffee with Ada.", "8/7/2018");
babbageCalendar.addEvent("Difference Engine presentation.", "8/2/2018");

babbageCalendar.listEvents();
  /*
    Logs:
    Charles Babbage's events are: 
    8/2/2018: Difference Engine presentation.
    8/7/2018: Coffee with Ada.
  */