JavaScript design pattern: difference between module pattern and revealing module pattern?

Question

I'm reading the book Learning JavaScript Design Patterns recently. What I don't get is the difference between module pattern and revealing module pattern. I feel they are the same thing. Anyone can give an example?

Answer 1

There are at least three different ways to implement the Module Pattern, but the Revealing Module Pattern is the only Module Pattern descendant that has an official name.

The Basic Module Pattern

The Module Pattern must satisfy the following:

• Private members live in the closure.
• Public members are exposed in the return object.

But there's a lot of ambiguity in this definition. By resolving the ambiguity differently, you get variants of the Module Pattern.

The Revealing Module Pattern

The Revealing Module Pattern is the most famous and most popular of the Module Pattern variants. It has a number of advantages over the other alternatives, such as

• Rename public functions without changing function body.
• Change members from public to private or vice versa by modifying a single line, without changing the function body.

The RMP satisfies three additional conditions in addition to those in the original:

• All members, whether public or private, are defined in the closure.
• The return object is an object literal with no function definitions. All right hand side expressions are closure variables
• All references are via the closure variables, not the return object.

The following example shows how it's used

var welcomeModule = (function(){
  var name = "John";
  var hello = function(){ console.log("Hello, " + name + "!");}
  var welcome = function() { console.log( hello() + " Welcome to StackOverflow!");}
  return {
    name: name,
    sayHello: hello,
    sayWelcome: welcome
  }
})();

If you wanted to make name and sayHello private, you just need to comment out the appropriate lines in the return object.

var welcomeModule = (function(){
  var name = "John";
  var hello = function(){ console.log("Hello, " + name + "!");}
  var welcome = function() { console.log( hello() + " Welcome to StackOverflow!");}
  return {
    //name: name,
    //sayHello: hello,
    sayWelcome: welcome
  }
})();

The Module Pattern with Object Literal

This is probably the oldest variant of the Module Pattern. Unlike RMP, there's no sexy official name for this variant.

It satisfies the following conditions, in addition to the original:

• Private members are defined in the closure.
• Public members are defined in the return object literal.
• References to public members are via this, whenever possible.

In the following example, you can see how, in contrast to RMP, the function definitions are actually in the return object literal, and references to members are qualified by this.

var welcomeModule = (function(){
  return {
    name: "John",
    sayHello: function(){ console.log("Hello, " + this.name + "!");}
    sayWelcome: function() { console.log( this.hello() + " Welcome to StackOverflow!");}
  }
})();

Note that unlike RMP, in order to make name and sayHello private, the references pointing to name and sayHello in the various function body definitions also have to be changed.

var welcomeModule = (function(){
  var name = "John";
  var sayHello = function(){ console.log("Hello, " + name + "!");};
  return {
    //name: "John",
    //sayHello: function(){ console.log("Hello, " + this.name + "!");}
    sayWelcome: function() { console.log( hello() + " Welcome to StackOverflow!");}
  }
})();

The Module Pattern with Return Object Stub

This variant also has no official name.

It satisfies the following conditions, in addition to the original:

• An empty return object stub is defined at the beginning.
• Private members are defined in the closure.
• Public members are defined as members of the stub
• References to public members are via the stub object

Using our old example, you can see that public members are directly added to the stub object.

var welcomeModule = (function(){
  var stub = {};
  stub.name = "John";
  stub.sayHello = function(){ console.log("Hello, " + stub.name + "!");}
  stub.sayWelcome = function() { console.log( stub.hello() + " Welcome to StackOverflow!");}
  return stub;
})();

If you want to make name and sayHello private as before, the references to the now-private members have to be changed.

var welcomeModule = (function(){
  var stub = {};
  var name = "John";
  var sayHello = function(){ console.log("Hello, " + name + "!");}
  
  stub.sayWelcome = function() { console.log( hello() + " Welcome to StackOverflow!");}
  return stub;
})();

Summary

The differences between the Revealing Module Pattern and the other variants of the Module Pattern is primarily in how public members are referenced. As a result, RMP is much easier to use and modify, which accounts for its popularity. However, these advantages come at a great cost (in my opinion), which Addy Osmani alludes to in his post on the Revealing Module Pattern,

A disadvantage of this pattern is that if a private function refers to a public function, that public function can't be overridden if a patch is necessary. This is because the private function will continue to refer to the private implementation and the pattern doesn't apply to public members, only to functions.

Public object members which refer to private variables are also subject to the no-patch rule notes above.

As a result of this, modules created with the Revealing Module pattern may be more fragile than those created with the original Module pattern, so care should be taken during usage.

and which I've talked about in some other posts.