QUESTION: What are mixins?

Mixins in JavaScript represent a design pattern used as an effective alternative to multiple inheritance, which is not directly supported by the language. The essence of a mixin is encapsulating a specific set of methods and properties into a separate object or function, which can then be added to the prototype of the target class.

The simplest way to implement a mixin is using the Object.assign(Class.prototype, mixin) method. This approach allows you to quickly copy all methods from the mixin object directly into your class's prototype, making them available to all future instances. For more complex scenarios, mixin functions are used, which take a base class as an argument and return a new extended class using the extends syntax with dynamic return values.

Using mixins allows you to build application architecture based on behavior composition rather than a rigid inheritance hierarchy. You can create small, independent modules of functionality—such as logging, event management, or validation—and attach them only to the classes that actually need them, avoiding code duplication and the creation of cumbersome inheritance trees.