In object-oriented design, one of the principle aims is to produce code that is flexible, maintainable, and reusable. One of the ways to do this is to use abstractions in your code rather than concretions. The more your objects know about how one another are implemented, the more dependencies there are in your system. As the number of dependencies grows, the potential for cascading breakage grows as well. But what happens when you have a system that requires certain objects to come from the same family?

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In some situations, it may be preferable not to mutate a given object. Or more likely, you won’t even have the option of mutating an object because it may come from a codebase over which you have no control (such as an external library). Such cases are relatively common; however, it’s still possible to enhance an immutable object’s behavior. One effective means to do so is with the decorator pattern.

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One of the benefits of object-oriented design is the ability for objects to share some behaviors while simultaneously differing in others. Typically, this is achieved through inheritance--when many subclasses inherit properties from a parent class but can optionally override certain behaviors as needed. This is a very useful and common design pattern; however, there are situations when polymorphism through inheritance is inappropriate.

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Decreasing coupling with the principle of least knowledge.

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The relationship between 'injecting' and 'inverting' dependencies in object-oriented design.

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As a JavaScript developer, much of the code you will be writing will deal with objects. We create objects to help organize code, reduce redundancy, and reason about problems using object-oriented techniques. The benefits of object-oriented design are readily apparent, but recognizing the usefulness of objects is just the first step. Once you have decided to use an object-oriented structure in your code, the next step is to decide *how* to do it.

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