Learning JavaScript Design Patterns

The Decorator Pattern

Decorators are a structural design pattern that aim to promote code reuse. Similar to Mixins, they can be considered another viable alternative to object subclassing.

Classically, Decorators offered the ability to add behavior to existing classes in a system dynamically. The idea was that the decoration itself wasn’t essential to the base functionality of the class; otherwise, it would be baked into the superclass itself.

They can be used to modify existing systems where we wish to add additional features to objects without the need to heavily modify the underlying code using them. A common reason why developers use them is that their applications may contain features requiring a large quantity of distinct types of object. Imagine having to define hundreds of different object constructors for, say, a JavaScript game (Figure 9-11).

Figure 9-11. Decorator pattern

The object constructors could represent distinct player types, each with differing capabilities. A Lord of the Rings game could require constructors for Hobbit, Elf, Orc, Wizard, Mountain Giant, Stone Giant, and so on, but there could easily be hundreds of these. If we then factored in capabilities, imagine having to create subclasses for each combination of capability type—e.g., HobbitWithRing, HobbitWithSword, HobbitWithRingAndSword, and so on. This isn’t very practical and certainly isn’t manageable when ...