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Grokking the Frontend System Design Interview

Component-Based Architecture in Frontend Systems

Explore the fundamentals of component-based architecture in frontend systems, including reusable UI components, lifecycle management, and data flow. Understand effective component design principles, hierarchy organization, and how to manage interactions. Learn the difference between component-based and micro-frontend architectures while recognizing challenges like prop drilling and state management to build scalable and maintainable frontend applications.

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In the early days of web development, UIs were often built monolithically; HTML, CSS, and JavaScript were entangled in a single file or loosely separated, often with repetitive code scattered across the app.

This led to common problems:

  • Making one UI change meant editing multiple files or duplicating code.

  • Reusing a UI pattern (like a button, input, or card) required copy-pasting.

  • Debugging became a nightmare in large codebases with no clear structure.

  • Consistency was hard to enforce, both visually and behaviorally.

As applications matured, developers needed a way to encapsulate UI behavior and styling into self-contained units, to build larger systems by composing smaller, reusable pieces like how microservices changed backend design.

This set the stage for a new approach to frontend design, one in which the UI isn’t a single large surface to be hacked together but a system of smaller, interchangeable parts that can be composed with purpose and reused intelligently, a component-based architecture (CBA).

Before we explore how this architecture works, let’s define what it is.

What is component-based architecture?

Component-based architecture (CBA) is a design approach in which a user interface is broken down into self-contained, reusable units called components. Each component encapsulates a specific piece of UI, including its structure (HTML/JSX), behavior (JavaScript/logic), and styling (CSS), making it a single responsibility unit within the larger application.

Instead of building an interface top-down in one large file, CBA enables you to compose interfaces bottom-up, like building with LEGO blocks. You might start with small elements (buttons, input fields), group them into reusable components (forms, cards), and then nest them into higher-level views (pages, layouts).

For example, a simple Button component might look like this in React:

Javascript (babel-node)
function Button({ label, onClick }) {
  return (
    <button className="btn" onClick={onClick}>
      {label}
    </button>
  );
}

This can now be reused across the app with different labels or behaviors, maintaining visual and behavioral consistency. Components are composable, meaning large interfaces are built by nesting smaller components:

An example of components nested to form a page
An example of components nested to form a page

Component life cycle

Once we define components, we must understand their life cycle; components go through phases; they are created, updated, and eventually removed when no longer needed. Managing these transitions properly ensures smooth user experiences and optimal performance. A typical component life cycle consists of three key phases:

  1. Mounting (initialization): The component is born; it appears in the UI, loads data, and sets up interactions (e.g., fetching user info when a profile page loads).

  2. Updating (state/props changes): The component adapts; it reacts to user inputs, re-renders when needed, and keeps the UI dynamic (e.g., a ...