Debug Skill

Name: debug
Rating: 92
Author: AdemKao

Instructions

•
Reproduce the Issue
- •Get exact steps to reproduce
- •Identify inputs that cause the bug
- •Confirm it's consistently reproducible

•

Gather Information

bash

# Check error logs
cat logs/error.log | tail -50

# Check recent changes
git log --oneline -10
git diff HEAD~5

•
Form Hypothesis
- •What could cause this behavior?
- •List 2-3 most likely causes
- •Rank by probability
•
Isolate the Problem
- •Binary search through code
- •Add logging/breakpoints
- •Create minimal reproduction
•
Identify Root Cause
- •Don't stop at symptoms
- •Ask "why" multiple times
- •Find the actual bug, not a workaround
•
Fix the Bug
- •Fix the root cause
- •Add test to prevent regression
- •Verify fix doesn't break other things

•

Verify Fix

bash

pnpm test
pnpm build
# Manual testing if needed

Debugging Techniques

Add Strategic Logging

typescript

// Temporary debug logging
console.log("[DEBUG] Function called with:", { userId, options });
console.log("[DEBUG] Query result:", result);
console.log("[DEBUG] State before update:", this.state);

// Remember to remove before commit!

Binary Search

typescript

// 1. Find approximate location
// 2. Add log in middle
// 3. Is bug before or after?
// 4. Repeat until found

function processData(data: Data[]) {
  console.log("[DEBUG] 1. Start, data length:", data.length);

  const filtered = filterData(data);
  console.log("[DEBUG] 2. After filter:", filtered.length);

  const transformed = transformData(filtered);
  console.log("[DEBUG] 3. After transform:", transformed.length);

  const result = aggregateData(transformed);
  console.log("[DEBUG] 4. After aggregate:", result);

  return result;
}

Check Assumptions

typescript

// Verify your assumptions
function getUser(id: string) {
  console.assert(typeof id === "string", "id should be string");
  console.assert(id.length > 0, "id should not be empty");

  const user = db.users.find(id);
  console.assert(user !== undefined, "user should exist");

  return user;
}

Minimal Reproduction

typescript

// Create isolated test case
it("should reproduce the bug", () => {
  // Minimal setup
  const input = { userId: "123", status: "active" };

  // Call the function
  const result = problematicFunction(input);

  // This fails - bug reproduced
  expect(result).toBe(expectedValue);
});

Common Bug Categories

1. Off-by-One Errors

typescript

// ❌ Bug: Missing last element
for (let i = 0; i < array.length - 1; i++)

// ✅ Fix
for (let i = 0; i < array.length; i++)
// or
for (let i = 0; i <= array.length - 1; i++)

2. Null/Undefined

typescript

// ❌ Bug: Accessing property of undefined
const name = user.profile.name;

// ✅ Fix: Optional chaining + fallback
const name = user?.profile?.name ?? "Unknown";

3. Async Issues

typescript

// ❌ Bug: Not awaiting async function
function getData() {
  const data = fetchData(); // Missing await
  return data.items; // data is Promise, not result
}

// ✅ Fix
async function getData() {
  const data = await fetchData();
  return data.items;
}

4. State Mutation

typescript

// ❌ Bug: Mutating state directly
function addItem(items: Item[], newItem: Item) {
  items.push(newItem); // Mutates original
  return items;
}

// ✅ Fix: Return new array
function addItem(items: Item[], newItem: Item) {
  return [...items, newItem];
}

5. Scope/Closure Issues

typescript

// ❌ Bug: Loop variable captured by reference
for (var i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 100);
}
// Prints: 3, 3, 3

// ✅ Fix: Use let or IIFE
for (let i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 100);
}
// Prints: 0, 1, 2

Debug Report Format

markdown

## Bug Report: [Brief Description]

### Symptoms

- What user observed
- Error messages

### Reproduction Steps

1. Step 1
2. Step 2
3. Bug occurs

### Root Cause

[Explanation of why the bug occurred]

### Fix

[Description of the fix]

### Files Changed

- `src/module/file.ts` - [what changed]

### Tests Added

- `src/module/file.test.ts` - [test description]

### Verification

- [x] Bug no longer reproduces
- [x] All tests pass
- [x] No regression in related features

Anti-Patterns

❌ Don't Do This

•Fix symptoms without understanding cause
•Make multiple changes at once
•Skip adding regression tests
•Leave debug logging in code
•Assume you know the cause without verification

✅ Do This

•Reproduce before fixing
•One change at a time
•Verify each hypothesis
•Add test that would have caught the bug
•Clean up debug code