Composing Software

Why Composition is Harder with Classes

Previously, we examined factory functions and looked at how easy it is to use them for composition using functional mixins. Now we’re going to look at classes in more detail, and examine how the mechanics of class get in the way of composition.

We’ll also take a look at the good use-cases for classes and how to use them safely.

ES6 includes a convenient class syntax, so you may be wondering why we should care about factories at all. The most obvious difference is that constructors and class require the new keyword. But what does new actually do?

• Creates a new object and binds this to it in the constructor function.
• Implicitly returns this, unless you explicitly return another object.
• Sets the instance ...