Reading Other People's Code

Debugging Code You've Never Seen/Assessment

Practice & Assessment

Test your understanding of Debugging Code You've Never Seen

Multiple Choice Questions

A stack trace has 15 frames. The first 2 are in your application code and the rest are in Express internals. Where is the bug most likely located?

What is the key difference between Step Over and Step Into in a debugger?

You're debugging a bug that only occurs for users with a specific ID. Which debugger feature handles this most efficiently?

In production logs, what is the primary purpose of a requestId or correlationId field?

When forming a debugging hypothesis, what makes it 'good' versus 'bad'?

Coding Challenges

Stack Trace Detective

Create a Node.js file buggy-app.js containing three functions: parseConfig(filePath), validateConfig(config), and startApp(filePath) that calls the first two. Introduce a bug where parseConfig returns null for a missing file, and validateConfig crashes with a TypeError when accessing null.timeout. Run the app to generate a stack trace. Then write a bug-report.md containing: (1) the full stack trace, (2) identification of the application frame where the bug originates, (3) your diagnosis of the root cause, (4) the fix applied, (5) the corrected output. Input: your buggy-app.js. Output: bug-report.md and fixed-app.js. Time estimate: 20 minutes.

Easy

Mini Project

Debugging an Open-Source Bug

Go to the GitHub issues list of any popular Node.js or Python library with 1000+ stars. Find an open bug labeled 'bug' that has a reproduction case. Your task: (1) reproduce the bug locally and capture the full stack trace in reproduction.txt, (2) use the debugger (not just console.log) to trace execution to the root cause — document your session as a step-by-step walkthrough in debug-session.md, (3) form three successive hypotheses, note which were wrong and why in hypotheses.md, (4) identify the exact line and variable state where behavior diverges from expected, (5) write a proposed fix in fix.patch (git diff format) and explain it in fix-explanation.md. If you can, open a real pull request. Uses all Module 4 skills: stack trace reading, strategic logging, debugger usage, hypothesis formation, and log analysis.

Hard