Notes for lecture 10: Scope, Module Pattern, and the Singleton Pattern

lecture10.notes.igm330.2017.markdown

Start of class:

• Quiz #4

Design Patterns

• First, what is a design pattern?

  • Original idea came from architect Christopher Alexander, who described a a series of patterns used in architecture and civic planning in his book "A Pattern Language" (https://www.amazon.com/Pattern-Language-Buildings-Construction-Environmental/dp/0195019199/) http://www.iwritewordsgood.com/apl/set.htm

• In 1994, The "Gang of Four" released "Design Patterns", a similar concept but for software engineering.

• There are now design pattern books many programming languages, including JavaScript: https://addyosmani.com/resources/essentialjsdesignpatterns/book/

• Today we'll look at the Module pattern and the Singleton pattern, because both of them make specific use of closures in JS, which we read about this past week.

Module Pattern

• Goals:
  • Encapsulate data... no polluting of namespaces
  • Control read / write access to data via closures and dedicated functions

var Module = function( arg1, arg2 ) {
  var data1 = arg1,
      data2 = arg2
      
  var instance = {
    getData1: function()  { return data1 },
    setData1: function(v) { data1 = v },
    getData2: function()  { return data2 },
    setData2: function(v) { data2 = v },    
  }
  
  return instance
}

var myinstance = Module( 1,2 )
console.log( myinstance.data1 ) // undefined
console.log( myinstance.getData1() ) // 1

Singleton Pattern

• Goals:
  • Instantiate an object only once, usually after some type of initialization condition has been met, perhaps in a window.onload callback function.
  • Always return the same object instance.

var Singleton = function() {
  var obj = null
  
  var init = function() {
    if( obj === null ) {
      obj = {
        foo: Math.random()
      }
    }
    
    return obj
  }
  
  return init
}

var constructor = Singleton()
var singleton1 = constructor()
var singleton2 = constructor()

console.log( singleton1.foo === singleton2.foo )