Module 3: Boolean Logic Testing

Boolean logic bugs are the most dangerous kind. The system works for 95% of users. It accepts most inputs correctly. It rejects most invalid ones correctly. And then, for exactly the right combination of edge conditions, it does exactly the wrong thing — silently.

The reason? A && that should be ||. A precedence assumption without parentheses. A NOT applied to the wrong expression. These are the bugs that survive code review and unit tests and only show up when a real user finds them.

Lessons in this module

1. AND, OR, NOT — Where Developers Go Wrong — Survey the most common boolean logic mistakes: using && when || was intended, inverting the wrong condition, and confusing short-circuit evaluation. See each mistake as a test case.
2. Operator Precedence Traps — In most languages, && binds tighter than ||. What looks unambiguous to the developer is actually A || (B && C), not (A || B) && C. Learn to spot these traps from a black-box perspective.
3. Testing Compound Conditions Systematically — Apply a lightweight version of Modified Condition/Decision Coverage (MC/DC): the technique that forces every condition to independently affect the outcome. Build test sets that guarantee you exercise every logical path.

Practice: Promo Code Engine

A promo code that should activate when (amount > 100 AND day == Saturday) OR user == new. But the developer wrote something subtly different. Your job: find the logical breach.

By the end of this module, you will earn the Logic Gate Guardian badge.