Understanding React Class Components: Why Old Codebases No Longer Scare Me 🤫

Source: Dev.to

Overview

Today’s learning was not about writing new React code.
It was about understanding old React code.

I already knew JavaScript classes. I had studied them before, so class components were not something “new” in terms of syntax. But when I started reading React class components, I realized:

This is not just JavaScript classes. This is React’s old mental model built on top of classes.

And that’s what takes time to understand.

When I first looked at class components, I didn’t think, “What is this new thing?” I thought, “Okay, this is just JS classes used inside React.” And that is true.
But React adds its own concepts on top:

• this.state
• this.setState
• Lifecycle methods like componentDidMount, componentDidUpdate, componentWillUnmount

So even though the syntax felt familiar, the React behavior model was different. The real challenge was not learning classes, but how React uses them.

Mapping Old Ideas to New Ones

The biggest clarity came when I started mapping old ideas to new ones:

• useState ≈ this.state and this.setState
• useEffect ≈ lifecycle methods
• Cleanup function ≈ componentWillUnmount

Then it clicked: Hooks did not invent new concepts; they simplified and unified old ones. Understanding this made both old React and new React much clearer.

Why Understanding Old React Matters

In real jobs:

• You won’t always work on new projects.
• You will often join existing codebases, many of which still use class components.

If you don’t understand them, you can’t confidently:

• Read code
• Debug issues
• Refactor safely

Takeaways

• Understanding old React is part of being a real React developer, not just a tutorial one.
• It’s not about going backward; it’s about understanding the foundation of modern React.
• Knowing how React used JavaScript classes before hooks makes me:
  • More confident reading old code
  • More comfortable in real‑world projects
  • More mature as a React developer

Still learning. Still improving. Still building. 💪