Code Quality Foundations

The Four Goals of High-Quality Code

Goal	Question to Ask	Failure Mode
It works	Does it solve the problem correctly?	Bugs, unhandled edge cases, unmet requirements
It keeps working	Will it survive changes around it?	Brittle dependencies, no tests, hidden assumptions
It's adaptable	Can requirements change without a rewrite?	Rigid coupling, over-engineering, hardcoded assumptions
It doesn't reinvent the wheel	Are we reusing proven solutions?	Custom code for solved problems, duplicated effort

The Six Pillars of Code Quality

Pillar	What It Means	Key Technique
Readable	Other engineers can understand it quickly	Descriptive names, clean layers, consistent style
No surprises	Behavior matches expectations (POLA)	Explicit contracts, no magic values, no hidden side effects
Hard to misuse	Difficult to use incorrectly	Type safety, immutability, validated construction
Modular	Components can be swapped independently	Interfaces, DI, separation of concerns, cohesion
Reusable & generalizable	Solves problems broadly, not just one case	Focused parameters, generics, avoiding assumptions
Testable	Can be verified in isolation	Modularity enables testability; design for it from the start

POLA = Principle of Least Astonishment. If a function does something a reasonable caller wouldn't expect, it's a bug in the design, even if it "works."

Modern Additions (2024-2026 Industry Consensus)

Concern	Why It's Now Explicit	Original Coverage
Secure	Shift-left security; threat model at design time	Implicit in "works"
Efficient	Resource optimization is a design choice, not afterthought	Subsumed under "works"
Reliable	Explicit error recovery and graceful degradation	Subsumed under "keeps working"

Decision Tables

"Should I Abstract This?"

Signal	Action
Same logic appears in 2+ places	Extract to shared function/class
Function can't be described in one sentence	Split into smaller functions
Class has method groups that use different fields	Consider splitting into separate classes
Changing one feature requires touching many files	Improve modularity and layer boundaries
Only one use case exists	Wait — don't abstract prematurely

"Where Should I Invest Quality Effort?"

Situation	Focus On	Rationale
Greenfield project	Modularity, clean interfaces	Foundation decisions compound; structure early
Legacy codebase	Testing, readability	Understand before changing safely
High-traffic service	Reliability, efficiency, testing	Production failures are expensive
Prototype / spike	Working code, speed	Validate the idea; plan to rewrite
Shared library / API	Hard to misuse, no surprises	You can't predict how consumers will use it
Security-sensitive code	Security, testability	Breaches are catastrophic and trust-destroying

Checklists

Before Writing Code

• Problem clearly understood (requirements, constraints, edge cases)
• Existing solutions checked (libraries, shared code, prior art)
• Testing strategy considered (how will this be verified?)
• Key tradeoffs identified (what are we choosing, and what are we giving up?)

Code Quality Self-Review

• Functions translate to single sentences
• No surprises: behavior matches names and types
• Hard to misuse: invalid states are unrepresentable where possible
• Modular: changes are localized, not scattered
• Testable: can be unit tested without complex setup
• Clean abstractions: API doesn't leak implementation details