How to Read Code You Didn't Write

Reading unfamiliar code is a skill. Here's how to get better at it.

Start With the Big Picture

Before diving into files, understand what the code does:

1. Read the README - What problem does it solve?

2. Check the package.json/requirements.txt - What dependencies give hints about functionality?

3. Look at the folder structure - How is it organized?

src/ components/ # UI pieces services/ # Business logic utils/ # Helper functions api/ # Backend communication

Find the Entry Point

Every program has a starting point:

- Web apps: index.html, App.js, main.ts

- APIs: server.js, app.py, main.go

- CLI tools: bin/ folder, main() function

Start there and follow the flow.

Read Top-Down, Then Depth-First

1. Skim the file - Get function names, class names, imports

2. Read public interfaces - What does this module expose?

3. Dive into specifics - Only when you need to understand implementation

// First pass: "This file handles user authentication"
export class AuthService {
  login(email, password) { ... }
  logout() { ... }
  refreshToken() { ... }
}
// Second pass: "How does login actually work?"

Trace the Data Flow

Pick a feature and follow data through the system:

1. Where does input come from? (UI, API, file)

2. How is it transformed?

3. Where does output go?

This reveals the architecture faster than reading file-by-file.

Use Your Tools

- Search globally - Find where functions are called

- Go to definition - Jump to implementations

- Find references - See all usages

- Git blame - Understand why code exists

Ask the Right Questions

Instead of "What does this do?", ask:

- "What problem does this solve?"

- "What would break if I removed this?"

- "What are the inputs and outputs?"

Write Notes as You Go

Auth Flow 1. User submits login form → LoginPage.tsx 2. Calls authService.login() → services/auth.ts 3. Posts to /api/auth → server/routes/auth.js 4. Returns JWT, stored in localStorage

Your future self will thank you.

When You're Stuck

- Run the code - Debuggers reveal flow

- Write a test - Forces you to understand interfaces

- Add console.logs - Trace execution path

- Ask the author - If available, they know the "why"

Key Takeaway

Reading code is detective work. Don't try to understand everything at once. Follow the trail that matters for your current task.