trace-and-isolate

name	trace-and-isolate
description	Applies systematic tracing and isolation techniques to pinpoint exactly where a bug originates in code. Use when a bug is hard to locate, code is not working as expected, an error or crash appears with unclear cause, a regression was introduced between recent commits, or you need to narrow down which component, function, or line is faulty. Covers binary search debugging, git bisect for regressions, strategic logging with [TRACE] patterns, data and control flow tracing, component isolation, minimal reproduction cases, conditional breakpoints, and watch expressions across TypeScript, SQL, and bash.
version	1.0.0
triggers	["trace execution","isolate bug","narrow down","binary search debugging","find where"]
tags	["debugging","tracing","isolation","bisect"]
difficulty	intermediate
estimatedTime	15
relatedSkills	["debugging/root-cause-analysis","debugging/hypothesis-testing"]

Trace and Isolate

You are using systematic tracing and isolation techniques to narrow down where a bug originates. The goal is to find the exact location in code where behavior diverges from expectation.

Quick Reference

Scenario	Technique
Large codebase / complex flow	Binary search debugging
Bug appeared between commits	git bisect
Unclear data transformation	Strategic [TRACE] logging
Which component is faulty?	Component isolation + mocks
Bug hard to reproduce	Minimal reproduction case
Conditional or intermittent bug	Conditional breakpoints / watch expressions

Binary Search Debugging

When you have a large codebase or complex flow:

1. Identify the start - Last known good state
2. Identify the end - First observed bad state
3. Test the middle - Check if behavior is good or bad
4. Repeat - Binary search on the half with the bug

Code Path Binary Search

Start: User clicks submit button
End: Error shown to user

Midpoint 1: Form validation
→ Data looks correct here? Continue to later code
→ Data already wrong? Focus on earlier code

Midpoint 2: API request construction
→ Request payload correct? Focus on server side
→ Payload already malformed? Focus on form handling

...Continue until exact line is found

Git Bisect for Regressions

When a bug appeared between two commits:

# Start bisect
git bisect start

# Mark current (broken) state as bad
git bisect bad

# Mark last known good state
git bisect good v2.3.0

# Git checks out middle commit, test and mark
git bisect good  # or git bisect bad

# Repeat until found
# Git will report: "abc123 is the first bad commit"

# Clean up
git bisect reset

Automated bisect with test script:

git bisect start HEAD v2.3.0
git bisect run npm  -- --grep