Systematic Debugging

Overview

Random fixes waste time and create new bugs. Quick patches mask underlying issues.

Core principle: ALWAYS find root cause before attempting fixes. Symptom fixes are failure.

The Iron Law

NO FIXES WITHOUT ROOT CAUSE INVESTIGATION FIRST

If you haven't completed Phase 1, you cannot propose fixes.

When to Use

Use for ANY technical issue:

• Test failures (unit, integration, browser mode)
• Flaky tests (pass sometimes, fail under load)
• Vue reactivity issues
• Dexie/IndexedDB problems
• Build failures
• Type errors

Use ESPECIALLY when:

• Under time pressure (emergencies make guessing tempting)
• "Just one quick fix" seems obvious
• You've already tried multiple fixes
• Previous fix didn't work

The Four Phases

Phase 1: Root Cause Investigation

BEFORE attempting ANY fix:

1. Read Error Messages Carefully

   • Full stack traces, not just the first line
   • Vitest output includes file paths and line numbers
   • TypeScript errors show the full type mismatch

2. Reproduce Consistently

   # Run specific test
   pnpm test src/features/workout/__tests__/specific.test.ts
   
   # Run with verbose output
   pnpm test --reporter=verbose
   

3. Check Recent Changes

   git diff HEAD~5 --stat
   git log --oneline -10
   

4. Gather Evidence in Multi-Component Systems

   For Vue/Dexie issues, trace through layers:

   Component → Composable → Repository → Dexie → IndexedDB
   

   Add console.error at each layer to find where it breaks.

5. Trace Data Flow

   See references/root-cause-tracing.md for the complete technique.

Phase 2: Pattern Analysis

1. Find Working Examples

   • Similar working tests in the codebase
   • Same component patterns that work

2. Compare Against References

   • Check testing-conventions skill for test patterns
   • Check vue-integration-testing for browser mode patterns
   • Check repository-pattern for Dexie patterns

3. Identify Differences

   • What's different between working and broken?
   • Missing await? Missing resetDatabase()?
   • Wrong query priority (getByRole vs getByTestId)?

Phase 3: Hypothesis and Testing

1. Form Single Hypothesis

   • "I think X is the root cause because Y"
   • Write it down, be specific

2. Test Minimally

   • ONE change at a time
   • Don't fix multiple things at once

3. Verify Before Continuing

   • Did it work? Yes -> Phase 4
   • Didn't work? Form NEW hypothesis
   • DON'T add more fixes on top

Phase 4: Implementation

1. Create Failing Test Case

   • Use vue-integration-testing skill patterns
   • Query priority: getByRole > getByText > getByTestId

2. Implement Single Fix

   • ONE change addressing root cause
   • No "while I'm here" improvements

3. Verify Fix

   pnpm test           # All tests pass
   pnpm type-check     # No type errors
   pnpm lint           # No lint errors
   

4. If Fix Doesn't Work

   • STOP after 3 failed attempts
   • Question the architecture, not just the symptom
   • Discuss with user before attempting more fixes

Common Issues in This Stack

Flaky Tests

Symptom: Test passes sometimes, fails in CI or under load.

Common causes:

• Missing await on async operations
• Using arbitrary timeouts instead of expect.poll()
• Test isolation issues (missing resetDatabase())
• Race conditions in Vue reactivity

See: references/condition-based-waiting.md

Database Isolation Failures

Symptom: Test passes alone, fails when run with others.

Root cause: Data from other tests polluting state.

Fix pattern:

beforeEach(async () => {
  await resetDatabase()  // ALWAYS first
})

See: references/defense-in-depth.md

Vue Reactivity Issues

Symptom: State updates but UI doesn't reflect changes.

Debug pattern:

import { nextTick } from 'vue'

// After state change
await nextTick()
// Now check DOM

Red Flags - STOP and Follow Process

If you think:

• "Quick fix for now, investigate later"
• "Just try changing X and see"
• "Add multiple changes, run tests"
• "I don't fully understand but this might work"

STOP. Return to Phase 1.

Related Skills

• testing-conventions - Query priority, expect.poll(), gotchas
• vue-integration-testing - Page objects, browser mode patterns
• vitest-mocking - Test doubles and mocking patterns
• repository-pattern - Dexie/database patterns

Supporting Techniques

In references/ directory:

• root-cause-tracing.md - Trace bugs backward through call stack
• defense-in-depth.md - Validate at multiple layers
• condition-based-waiting.md - Replace timeouts with expect.poll()

In scripts/ directory:

• find-polluter.sh - Find which test creates pollution

Quick Reference