Learning JavaScript Design Patterns

The Factory Pattern

The Factory pattern is another creational pattern concerned with the notion of creating objects. Where it differs from the other patterns in its category is that it doesn’t explicitly require the use of a constructor. Instead, a Factory can provide a generic interface for creating objects, where we can specify the type of factory object we wish to be created (Figure 9-9).

Figure 9-9. Factory pattern

Imagine that we have a UI factory where we are asked to create a type of UI component. Rather than creating this component directly using the new operator or via another creational constructor, we ask a Factory object for a new component instead. We inform the Factory what type of object is required (e.g., “Button”, “Panel”) and it instantiates this, returning it to us for use.

This is particularly useful if the object creation process is relatively complex—e.g., if it strongly depends on dynamic factors or application configuration.

Examples of this pattern can be found in UI libraries such as ExtJS, where the methods for creating objects or components may be further subclassed.

The following is an example that builds upon our previous snippets using the Constructor pattern logic to define cars. It demonstrates how a vehicle factory may be implemented using the Factory pattern:

// Types.js - Constructors used behind the scenes

// A constructor for defining new cars
function ...