Dependency inversion and dependency injection are fundamental concepts in software development that enhance the modularity, maintainability, and testability of applications. While often associated with back-end projects, these principles hold significant importance in the context of front-end projects as well.
In this article, we will explore what dependency inversion and dependency injection are, and how to effectively apply them in your front-end projects using TypeScript, React.js, and Next.js.
Dependency inversion is a key principle of software development that involves reversing the flow of control within an application. Instead of a structure where low-level modules depend on high-level modules, dependency inversion advocates that low-level modules are independent and dependencies are provided externally.
In the front-end context, this means that components should not directly depend on external services, but rather depend on interfaces (or types) that we define as needed. This improves application flexibility and facilitates testing by allowing real dependencies to be easily substituted with simulated dependencies during testing.
This diagram illustrates the words of this article as well as the principle of dependency inversion in front-end with React or Nextjs:
Now that we have seen the theoretical part of front-end dependency inversion and injection, we will see how to apply it in a front-end project using TypeScript, React.js and Next.js.
TypeScript offers strong typing which can be used to define clear and precise interfaces for dependencies. You can use interfaces or types. Defining interfaces for dependencies makes it easier to substitute real dependencies with simulated dependencies during testing.
For the example project, we will develop a feature that allows you to connect.
In this example, the HTTP request to the API that allows the user to log in is directly in a React component. The problem with this practice is that we cannot easily test this component. Indeed, we cannot simulate the HTTP request to test the behavior of the component in different scenarios (for example, if the request fails).
Likewise, our React component, i.e. our user interface, is directly tied to the API request. If we want to change the API to connect, we need to modify our React component. This can be problematic if we have multiple components that depend on this API.
There is also another problem, if we want to add a new feature which requires login, we have to rewrite the login code in each component. This can quickly become problematic if we have several components that require connection.
Finally, our UI (the React components) contains business logic, which is not a good practice. This is because our React component should only focus on the UI and should not contain business logic. This makes our component harder to maintain and test.
So, to make the API call, just use the interface in your code. This prevents your components from depending on external libraries or services. Here is an example of a React component that uses the interface defined above for dependency inversion:
Inversion and dependency injection are practices that bring many benefits to development projects. Applying these principles using TypeScript, React.js, and Next.js in front-end projects allows you to build applications that are modular, easily testable, and scalable.
By reducing coupling with external libraries and services, these approaches improve code quality and facilitate long-term maintenance. These practices are also at the heart of French architecture (clean architecture). If you want to know more about it, I invite you to read the article I wrote about it by clicking on this link: see the article on the front-end hexagonal architecture.