Learning JavaScript Design Patterns

The Constructor Pattern

In classical object-oriented programming languages, a constructor is a special method used to initialize a newly created object once memory has been allocated for it. In JavaScript, as almost everything is an object, we’re most often interested in object constructors.

Object constructors are used to create specific types of objects—both preparing the object for use and accepting arguments a constructor can use to set the values of member properties and methods when the object is first created (Figure 9-1).

Figure 9-1. Constructor pattern

Object Creation

The two common ways to create new objects in JavaScript are as follows:

// Each of the following options will create a new empty object:

var newObject = {}; 

// or which is a shorthand for the object constructor
var newObject = new Object();

Where the Object constructor creates an object wrapper for a specific value, or where no value is passed, it will create an empty object and return it.

There are then four ways in which keys and values can be assigned to an object:

// ECMAScript 3 compatible approaches

// 1. Dot syntax

// Set properties
newObject.someKey = "Hello World"; 

// Get properties
var key = newObject.someKey; 



// 2. Square bracket syntax

// Set properties
newObject["someKey"] = "Hello World"; 

// Get properties
var key = newObject["someKey"]; 



// ECMAScript 5 only compatible approaches
// For more information ...