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JavaScript Design Patterns: Module, Singleton

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You can read about the evolution of these patterns at KlausKomenda, along with an example that solves the same task using a number of different patterns so you can see the progression. The patterns described here are the Module and the Singleton.

The Module (more specifically, the Revealing Module) is a design pattern that provides encapsulation for data members and/or functions. Why do we care? Here are some benefits:

  • Encapsulation avoids naming conflicts with other code on the page.
  • Encapsulated data can only be accessed via public methods (if provided), which lets the module perform any necessary validation before changing data. This is not "hack proof", nor does it intend to be. It simply means that private data is scoped to the module itself, and does not exist in the "public" world. See the following code for examples.
  • Encourages coding behavior that produces high cohesion and loose coupling.
  • Improves code readibility and reuse. You can often copy well-written modules to other projects without changing a line of code.
    var RevealingModule = function() {
        
        // private variables and functions go here
        var myPrivateVar = 'test';
        function getVar() {
            return myPrivateVar;
        }
        function setVar(newVar) {
            myPrivateVar = newVar;
        }

        return {
            // public methods are revealed here
            getVar: getVar,
            setVar: setVar
        };
         
    };
    
    // each instance gets its own copies of private data
    var modFirst = new RevealingModule();
    var modSecond = new RevealingModule();
    
    // changing one instance does not affect other instances
    modSecond.setVar('new value');
    alert(modSecond.getVar()); // 'new value'
    alert(modFirst.getVar());  // 'test'

A Singleton is an object that only has one instance. The easy way to accomplish this in JavaScript is to start with the Module pattern, then immediately instantiate it by using inline execution.

    // Dude looks like a Module...
    var Dude = function() {
        // private members and functions
      var name = "";
      var age = 0;
      function modifyAge(years) {
        age = age + years;
      }
      
      // these methods will be revealed below
      function getAge() { return age; }
      function getName() { return name; }
      function setAge(newAge) { age = newAge; }
      function setName(newName) { name = newName; }
      
      return {
        getName: getName,
        setName: setName,
        getAge: getAge,
        setAge: setAge,
        
        // public names do NOT have to match private names
        changeName: setName,
        
        // you can also return additional public methods
        haveBirthday: function() {
          modifyAge(1);
          alert(name + ' is now ' + age + ' years old!');
        },
        sayHello: function() {
          // other public methods can be called using "this"
          alert(this.getName() + ' says hello!');
        }
      }
      
    }(); // inline execution turns this Module into a Singleton
    
    // cannot instantiate Dudes, there can be only one
    var myDude = new Dude(); // TypeError: Dude is not a constructor
    
    /*
     * We can't make a new instance using "var myDude = new Dude();"
     * because Dude is an Object, not a Function. Instead, access 
     * Dude's public functions directly, like this:
     */
    Dude.setName('John');
    Dude.setAge(36);
    Dude.haveBirthday(); // alerts 'John is now 37 years old!'
    
    // private data will not be overwritten, it is bound immediately at creation
    Dude.name = 'Jim';     // this makes a new, PUBLIC property called "name"
    alert(Dude.getName()); // 'John' (public method accessing private property)
    alert(Dude.name);      // 'Jim' (public property we just created)
    alert(Dude.age);       // 'undefined' (only exists in private scope)

Modules and Singletons can be great for code reuse, because they can provide a simple public interface which doesn't have to change when its inner workings change. Some Singleton examples:

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