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We're building tools that help teams migrate from React class components to functional components — starting with free education, moving toward powerful automation. Creator of Unclass, a CLI and analyzer to bring legacy code into the modern React era. I write weekly about React migration, code transformation, and automation with TypeScript.
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🛠️ Purpose: This guide is a foundational deep-dive into React’s internals, crafted for software artisans preparing to migrate class components to modern function components. We’ll walk through React’s core concepts step by step – from the Virtual DO...
🌟 Introduction This is the third article from the Unclass team, written with a strong sense of mentorship and care. In our last post, we discussed whether switching from class-based to functional components in React is the right choice. If you're he...
🌟 Introduction This is the second article in the React migration series by the Unclass team. In the first article, we introduced the goals of this series and outlined the Unclass project. This time, we're stepping back to ask a more fundamental ques...
✍️ Introduction Hi there! 👋I'm Daniel, and together with my friend Alberto, we’re the team behind a project called Unclass. We’ve been working in React for years — and we’ve experienced the frustration firsthand:migrating from class components to fu...
Unclass.dev everything that you need to know about how to migrate from react with classes to functions
5 posts
A practical, step-by-step guide to migrating from React class components to functional components.
Welcome back! If you've been following our series, you'll recall that our earlier articles were aimed at architects and managers planning large-scale React migrations. This article marks a shift—now we're diving deeper into the technical aspects for developers who will carry out the actual migration.
This article is the technical introduction to React migration. It lays the theoretical groundwork necessary for developers to deeply understand how React works—before any code is modified. The series will be split into two parts:
Theoretical articles: These explain internal mechanics, architecture, and concepts every developer should understand before migrating.
Practical articles: These will walk you through actual code transformations, refactoring patterns, and edge cases you’ll encounter.
As mentors and software craftsmen, our goal is to help you not just change code, but do it with clarity, confidence, and long-term quality. This guide is here to equip you with the knowledge you’ll need to build a successful migration—without unnecessary headaches.
React is a well-documented JavaScript libraries, and there are countless tutorials and courses online. Our goal is not to repeat these materials but to provide you with specific insights needed for a successful migration. We'll refer to external resources when necessary, but we'll focus on practical understanding, offering you the tools and mental models that truly assist during migration work.
Before diving deeper, let's clarify some crucial terms you'll encounter:
Fiber: React’s internal architecture for incremental rendering and reconciliation.
Reconciler: Determines what changes React needs to apply.
Scheduler: Decides when those changes should be executed.
At its heart, React applications are built from components—reusable blocks of UI that encapsulate their own logic and appearance. Before diving into class vs. functional components, it's essential to grasp React’s general lifecycle and how it handles components internally.
React’s operation involves three fundamental phases:
Trigger: Something initiates a component render (a state change, prop change, or context update, first render).
Render: React calculates the changes by invoking your components. This is a pure computation step—it builds a virtual DOM tree but does not touch the browser DOM yet.
“Rendering” is React calling your components.
useEffect) run after painting.📚 Resource: React Docs – Render and Commit
This design allows React to batch updates, defer low-priority work, and minimize reflows and repaints—especially important in React 18 with concurrent features.
The lifecycle of React components can be classified in many ways. However, to clearly understand components—regardless of whether they're classes or functions—we simplify it into three main phases:
Mounting: The initial phase of creation and insertion into the DOM.
Updating: Reacting to changes (state, props, or context) and re-rendering.
Unmounting: Removal from the DOM and necessary cleanup.
Think of this lifecycle as stages of an organism's life: birth, growth, reproduction, and death—each with specific responsibilities and tasks.
Both class and functional components coexist seamlessly within React, but there are important distinctions, especially in how each interacts with React’s latest features.
Class Components: Defined using ES6 classes. React creates an instance, stores it in a Fiber node, and manages lifecycle via methods like componentDidMount or componentWillUnmount.
Functional Components: Defined as pure functions. They don’t have an instance. Instead, React keeps a hooks list on the Fiber node to manage state (useState, useEffect, etc.).
⚠️ Functional components with Hooks are now the preferred style in the React ecosystem.
🚫 Class components cannot use Hooks and may not support future features like Server Components.
Understanding how these two models are implemented internally helps you:
Translate lifecycle logic accurately.
Understand performance behavior.
Make confident architectural decisions during migration.
Class Component:
class Greeting extends React.Component {
state = { name: 'John' };
componentDidMount() {
console.log('Component mounted!');
}
render() {
return <h1>Hello, {this.state.name}</h1>;
}
}
Functional Component:
function Greeting() {
const [name, setName] = useState('John');
useEffect(() => {
console.log('Component mounted!');
}, []);
return <h1>Hello, {name}</h1>;
}
When migrating to functional components and Hooks, follow these React-recommended best practices:
Incremental Migration: Gradually migrate components, avoid rewriting everything at once.
Testing: Robustly test components before and after migration to catch regressions.
Performance Monitoring: Profile components to detect performance regressions.
React's General Recommendations:
Keep components pure, side-effects should be managed explicitly.
Clearly define component boundaries and minimize component complexity.
Avoid unnecessary re-renders using techniques like memoization (React.memo, useMemo, useCallback).
Avoid these common misunderstandings:
"Hooks are just lifecycle methods with new syntax": They represent a different paradigm, focusing on state synchronization rather than lifecycle events.
"Functional components are inherently faster": Performance depends on component logic, not the component type itself.
"React skips rendering if props don’t change": React always calls render functions, though DOM updates occur only when changes happen.
In future articles, we'll explore:
Detailed internal workings of class components.
Lifecycle method mappings.
Functional component internals and migration tactics.
These articles will maintain a mentor-like mindset, prioritizing clarity, best practices, and quality.
🌟 Mentor’s Tip
React isn't just a UI library—it’s a mindset. Understanding React's core principles helps you write better, clearer, and more maintainable code. Invest in understanding first—your migration will thank you.
Happy learning!
Additional Resources:
React Docs – useEffect