Earlier, I talked about the basics of JavaScript Closure. In this post, lets continue to explore Closure with the help of some practical examples.

Before we begin, just to recap,

Closure encloses function and the set of variables that were in scope of the function when it was declared. The variables inside the closure kept alive as long as the function alive.

With that in mind, let’s make some closures.

1. Maintain State between function calls

Let’s say you have function add() and you’d like it to add all the values passed to it in several calls and return the sum. For example,

add(5); // returns 5
add(20); // returns 25 (5+20)
add(3); // returns 28 (25 + 3)

Of course, you can use a global variable in order to hold the total. But keep in mind that this dude will eat you alive if you (ab)use globals.

For scenarios like this, Closure is the best candidate for maintaining state between function calls without using globals. Let’s see how.

// Using IIFE, to not to pollute global namespace.
(function(){

  var addFn = function addFn(){
    // local to closure and hold the value inbetween multiple calls.
    var total = 0;
    return function(val){
      total += val;
      return total;
    }

  };

  var add = addFn();

  console.log(add(5)); // 5
  console.log(add(20)); // 25
  console.log(add(3)); // 28

}());

Run this example on JSFiddle.

2. Partial application, a.k.a Currying

Suppose you have a function that takes several arguments and you only know values for some of the arguments in the beginning. For this scenario, you can make use of Currying technique to pre-fill the values for known arguments and supply values for the rest of the arguments later.

Here’s an example, illustrating Currying using Closure.

Assume you have a showMessage() function that shows given message on screen with the given type and position. It takes three arguments. So, every time you want to call this function, you need to supply these three values.

(function(){
  function showMessage(type, position, message){
    // displays a message at position and sets it type (for CSS styling)
  }

  showMessage('error', 'top', 'Not good.');
  showMessage('info', 'top', 'You better know this.');

}());

What if you want to make this function call, simpler? What if you create two other methods, namely, showError() and showInfo() that prefill the message type and position and supply the actual message in a later point in time? Let’s Curry them.

The Curry function is taken from John Resig’s post (which btw is a good read about Currying).

(function(){

  // Lets add Curry method to Function so that we can call it on any function we want.
  Function.prototype.curry = function(){
    var fn = this, args = Array.prototype.slice.call(arguments);
    return function(){
      return fn.apply(this, args.concat(Array.prototype.slice.call(arguments)));
    };
  };

  // Core method.
  function showMessage(type, position, message){
    console.log('showing [' + message + '] of type [' + type + '] at [' + position + '].' );
  }

  // Create special versions of Core method using Currying.
  var showError = showMessage.curry('error', 'top');
  var showInfo = showMessage.curry('info', 'bottom');

  // Call our special methods.
  showError('Not good.');
  showInfo('You better know this.');
  
}());

Run this example of JSFiddle

3. Private methods in JavaScript?

Yes, we can emulate private methods in JavaScript using Closure. Let’e see how.

(function(){

  var makeCar = function(){

    // private variable
    var fuel = 0;

    // private method
    function burnFuel(){
      fuel-=10;
      console.log('Burned fuel [10]');
    }

    return {
      accelerate : function(){
        if(fuel > 0){
          burnFuel();
        }else{
          console.log('Out of gas. Fill now.');
        }
      },

      fillGas : function(gas){
        if(fuel <= 100){
          fuel += gas;
        }else{
          console.log('Reached capacity. Stop spilling.');
        }
      }
    }

  };

  var car = makeCar();
  car.accelerate(); // Out of gas. Fill now.
  car.fillGas(75);
  car.accelerate(); // Burned fuel [10]
  car.accelerate(); // Burned fuel [10]

}());

Run this example on JSFiddle

As you can see, the makeCar() function returns an object with two methods: accelerate and fillGas. These two methods has access to the private method burnFuel and private variable fuel. But the outer world can not directlty access these two.

So, with the help of closure you can simulate object oriented programming in JavaScript.

With that, I am concluding this post of Closure examples. Of course, these are not the only examples of Closures. There are lot many out there. Btw, If you have written closure for an interesting use case, feel free to share it in the comments section.