Boost Your Coding Efficiency by Mastering TypeScript Mixins

What is the purpose of TypeScript mixins?

When writing code using principles of object-oriented programming, there is a strong desire to make the code more reusable. Programmers don’t want to write the same methods multiple times; instead, they write them once and then reuse them wherever and whenever possible.

This is where TypeScript mixins can really come in handy. This design pattern extends classes without inheritance but rather composition, so the result is a single class with both sets of functionalities.

What are mixins?

As written in the introduction, Mixins are a design pattern that is used to add functionalities to an object or class.

They work by extending the current object or class and returning a new class instance with both sets of functionalities. Mixins require two things: a type and a factory function.

Mixin classes elevate TypeScript

To return the class as an extended instance of the current one with additional functionalities, TypeScript allows for a mixin class approach. This is great for developers who combine multiple classes or want to achieve multiple inheritances within their applications, as the returned new class allows exactly that.

By using mixins in TypeScript, you can extend a single class or multiple classes with additional functionalities without the constraints traditional inheritance might impose.

Gaining a knowledge of TypeScript's capabilities allows developers to write more efficient and maintainable code.

The foundation: Type and factory functions

First, a type constructor will be used as a base from which all classes will be extended. TypeScript documentation provides a fairly simple type that looks like this:

There are a few parts of this line of code that need to be discussed:

• new (...args: any[]) - Entry parameters are defined using the spread operator by design. This makes it possible for any number of parameters of any type to be passed. This even opens up the possibility of passing Mixins as input parameters to get an object or a class with multiple sets of functionalities.

• {} - The exit parameter is a newly created object with additional functionalities.

Second, the factory function is needed.

The function must receive the base class as an input parameter and return a newly created class with the extended functionalities defined in the function body. A similar example to the one provided in the TypeScript documentation would be:

In this example, exampleMixin is a function that creates a new class. This example receives a class as a parameter and returns a new class with another method called method_1.

The duck example - a practical example of mixins

Mixin constructors can be further extended with generics to better fit the application's needs. For example, here is a Duck class where Mixins should be designed so that they can only extend that class:

But with time, it comes across that some ducks can have blue feet, like the ones from the Galapagos. If this feature was implemented using Mixins, first, a type needs to be created, but this time, it can be specified to allow only the Duck class:

Next, what is needed is a mixin function that will return a new Duck class:

Now, when there is a need to create a new class that supports ducks with blue feet, one line of code solves all problems:

Looking at the Duck class again, the hasOrangeFeet() method is no longer applicable in all scenarios, so it could also be written as a mixin and used when needed.

You can use multiple mixins for increased flexibility

Extending the functionalities of a class can be done using multiple mixins. For example, if there exists a duck that has both orange and blue feet, the following line of code can be crafted:

Instantiating the class, both methods can be utilized without any issues:

Simplifying complex types with mixins

Mixins allow the creation of reusable components and simpler partial classes, enhancing the type system and promoting code reuse. For instance, a factory function can be designed to apply mixins, making work with two or more declarations easier.

This approach simplifies managing classes that might otherwise require complex inheritance patterns, thus solving a variety of common use cases in a more maintainable and scalable way. Mixins simplify complex types and allow for the dynamic addition of properties to classes, enhancing their functionality.

Mixins vs. inheritance: The differences

Unlike inheritance, which typically extends a single class, mixins offer a more flexible solution that can blend the functionalities of multiple classes.

Practical mixin applications

Extending a base class with mixins in real-world scenarios allows developers to build reusable components efficiently.

Consider a scenario where two classes must share a method of the same name but implement it differently. Through declaration merging, TypeScript enables a smooth mixin integration, ensuring that each class retains its unique implementation while sharing a common interface. By using mixins, developers can streamline their TypeScript files, ensuring that each file maintains clear and concise functionality.

What are the differences compared to inheritance?

Inheritance is a cornerstone of object-oriented programming. With the keyword extends, child classes can inherit properties and methods from their parent classes. The inherited methods are already implemented and can be employed without adding additional logic unless there is a need to override them.

Mixins approach to the diamond problem

Although extending other classes can be very useful and can result in efficient coding, the diamond problem presents a significant challenge.

This problem occurs when a child class needs to extend two classes that already extend the same parent class, potentially leading to conflicts in method implementations. Mixins offer a solution to this dilemma, allowing the same method to be implemented multiple times, with the most recently added implementation taking precedence.

Overriding methods with mixins

To illustrate, continuing with the duck feet colors example from the previous sections, both mixins can be extended to accommodate another method with the same name, for instance, getFeatherColours():

Using mixins and creating a new instance would result in different outcomes, depending on which mixin was last called to extend the class:

If a different implementation is desired, ensuring that the preferred method implementation is the last one added prevents it from being overridden:

Why use mixins instead of interfaces?

There are a couple of differences between interfaces and mixins. The first one is that using mixins extends the target class with implemented functionalities. The new functionalities just need to be called upon for use.

With interfaces, the story is different, as only the method names are defined, and each class implementing them needs to write its own logic. Another limitation is that only static or constant variables can be defined, so there is no option to implement a variable that will change throughout the implementation's lifecycle.

However, they are similar in one aspect: They can not declare private or protected variables because they do not define the scope of the class they extend.

Mixins and the TypeScript Compiler

The TypeScript compiler is pivotal in ensuring that mixins are correctly applied to classes, preserving type annotations, and facilitating compile-time checks. This process ensures that objects created through mixins maintain their integrity and behave as expected within the TypeScript type system, thereby enhancing code quality and runtime performance.

Wrapping things up

Mixins present a formidable strategy for writing reusable and flexible code. They grant developers a significant degree of freedom in TypeScript, allowing for the implementation of specific classes or the extension of any object or class that requires additional functionalities.

Understanding the process of creating types might seem daunting at first, but with practice, it becomes clearer, enabling the addition of more specific and robust concepts.

Mixins are incredibly straightforward to use with everything set up. Their result is a seamless class or object with extended functionalities that can operate without drawbacks and be adapted for nearly any situation.