The pull request comment was brutal in its simplicity: "Where's the documentation for this?"
I stared at my screen, indignant. I'd written a perfectly functional authentication service. The code was clean, the tests passed, the feature worked exactly as specified. What more did they want?
"The code is self-documenting," I replied, confident in my junior developer wisdom. "Good code doesn't need comments."
Six months later, at 2 AM, trying to debug that same authentication service while half the user base couldn't log in, I finally understood what my senior colleague had been trying to tell me. The code wasn't self-documenting. It was self-obscuring.
That night, as I frantically traced through my own logic trying to remember why I'd made certain decisions, I learned the difference between writing code that works and writing code that lasts. The difference isn't in the implementation—it's in the documentation. But not the documentation junior developers think they need to write.
The Documentation Paradox
Junior developers either write no documentation or write too much of the wrong kind. They'll spend hours crafting detailed comments explaining what every line does ("increment counter by 1") while completely ignoring the most important question: why does this code exist?
Senior developers have learned something painful: the hardest person to write documentation for is yourself six months from now.
When you're deep in the flow of writing code, everything seems obvious. The architecture makes perfect sense. The variable names are self-explanatory. The business logic follows naturally from the requirements. Of course you chose that algorithm—it was the most elegant solution.
But code exists in time, not just in space. It lives in a world of changing requirements, evolving systems, and rotating team members. The "obvious" decisions you made today become mysterious artifacts that someone else—or future you—will need to understand under pressure.
What Juniors Document vs What Seniors Document
Juniors document syntax. Seniors document decisions.
I've seen junior developers write README files that are essentially code tutorials: "To use this function, pass in a string parameter and it will return a formatted result." But they'll leave out crucial context: "This formatting function exists because the third-party API returns inconsistent date formats, and we need to normalize them before storing in the database."
The syntax documentation becomes obsolete the moment you change the function signature. The decision documentation remains valuable for years.
Juniors explain how things work. Seniors explain why they work that way.
Watch a junior developer write API documentation. They'll meticulously document every endpoint, parameter, and response format. They'll include example requests and curl commands. But they won't explain the design philosophy that shaped the API structure, or the tradeoffs they made between consistency and performance.
Senior developers document the invisible layer—the reasoning that led to the current implementation. They understand that future maintainers don't just need to know how to use the code; they need to know how to change it intelligently.
Juniors write documentation for users. Seniors write documentation for inheritors.
When junior developers think about documentation, they think about external users trying to integrate with their code. This leads to comprehensive but shallow documentation—lots of examples, but no depth about the internal model or system boundaries.
Senior developers write primarily for the person who will need to modify, extend, or debug the system. They document edge cases, performance characteristics, and failure modes. They explain which parts of the code are stable and which parts are likely to change.
The Architecture of Understanding
The best senior developers I know treat documentation as architecture for understanding. Just like system architecture, it needs to work at multiple levels of abstraction.
At the system level, they document the mental models that shaped major decisions. Why did we choose microservices over a monolith? What are the key invariants that must be maintained? Where are the system boundaries and why did we draw them there?
At the component level, they document the contracts and responsibilities. What does this service own? What are its dependencies? How does it fail and what happens when it does?
At the function level, they document the non-obvious. Not what the function does (that should be clear from the name and implementation), but the constraints, assumptions, and edge cases that aren't visible in the code.
This isn't about writing more documentation. It's about writing documentation that creates transferable understanding—the ability for someone else to make intelligent changes to the system without breaking everything.
Tools That Scale Understanding
Modern AI tools have changed how senior developers approach documentation, but not in the way most people expect. They don't use AI to write more documentation faster. They use it to write better documentation more thoughtfully.
When designing a complex system, senior developers might use Claude 3.7 Sonnet to help them think through different architectural approaches and document the tradeoffs. The AI becomes a rubber duck for design decisions—forcing them to articulate why they chose one approach over another.
For API documentation, they might use GPT-4o mini not to generate boilerplate examples, but to help them think through different user journeys and edge cases. What happens when rate limits are hit? How should clients handle partial failures? The AI helps surface questions that lead to more complete documentation.
The Document Summarizer becomes valuable for maintaining documentation over time—helping teams identify what's become outdated or what critical information is missing from existing docs.
But the key insight is this: AI tools are most valuable when they help you think more clearly about what needs to be documented, not when they automate the documentation itself.
The Economics of Documentation Debt
Junior developers see documentation as overhead—something that slows down shipping features. Senior developers understand that undocumented code is technical debt disguised as velocity.
Every decision left undocumented becomes a mystery that someone will need to solve later. Every architectural choice left unexplained becomes a potential source of misguided refactoring. Every edge case left uncommented becomes a bug waiting to resurface.
The compound interest on documentation debt is vicious. A system with poor documentation doesn't just slow down new team members—it slows down everyone, including the original authors. Knowledge gets lost, assumptions get forgotten, and simple changes become risky because no one understands the full implications.
Senior developers have felt this pain enough times to change their behavior. They've been burned by their own undocumented code enough to understand that the few minutes spent writing a decision record or architectural note will save hours of investigation later.
The Human Layer
The most important thing senior developers document isn't technical at all—it's human context.
Who made key decisions and why? Not for blame, but for consultation. When you need to change a complex algorithm, knowing who originally implemented it can save days of reverse engineering.
What were the constraints at the time? Code that looks obviously wrong today might have been obviously right under different circumstances. Documentation of historical context prevents well-intentioned "improvements" that break things in subtle ways.
What alternatives were considered? This is the most valuable documentation of all. Future maintainers don't just need to know what you chose—they need to know what you rejected and why. This prevents them from walking down the same dead-end paths you already explored.
The Document Types That Actually Matter
Senior developers focus their documentation efforts on a small set of high-leverage document types:
Decision records for significant architectural choices. Not just what was decided, but what options were considered and what factors drove the decision.
Runbooks for operational procedures. Not just how to deploy, but how to debug common failure modes and what to do when things go wrong.
System maps showing how components interact. Not just service diagrams, but data flow, dependency chains, and failure propagation paths.
Change logs that explain not just what changed, but why it changed and what the implications are for other parts of the system.
Each of these document types solves a specific problem that code alone cannot solve: they preserve the reasoning and context that shaped the current implementation.
The Maintenance Mindset
The biggest difference between junior and senior documentation habits isn't what they write—it's how they maintain what they've written.
Junior developers treat documentation as a one-time activity. They write it when the code is fresh, then never touch it again. The documentation slowly diverges from reality until it becomes actively misleading.
Senior developers understand that documentation is a living system that requires ongoing maintenance. They build documentation maintenance into their development workflow. When they change code, they update the relevant documentation. When they learn something new about the system's behavior in production, they capture that knowledge.
They also design documentation systems that make maintenance easier. They keep documentation close to the code it describes. They use tools that can automatically detect when documentation is out of sync with implementation. They write documentation in formats that can be easily searched and updated.
The Network Effect
The best-documented codebases aren't created by individuals—they're created by teams that have developed a shared culture around documentation. Senior developers don't just write good documentation; they create systems and norms that encourage others to do the same.
They establish patterns for common documentation needs. They create templates that make it easy to document decisions consistently. They build documentation review into the code review process. They celebrate good documentation the same way they celebrate good code.
Most importantly, they model the behavior they want to see. When junior developers see senior team members taking time to write thoughtful decision records or update runbooks after incidents, it signals that documentation isn't just busywork—it's part of professional software development.
The Long Game
Documentation is ultimately about respecting the future. It's about acknowledging that software systems outlive the people who build them, and that our responsibility extends beyond shipping features to enabling future maintainers.
Junior developers optimize for shipping code quickly. Senior developers optimize for shipping code that can be maintained, extended, and understood over time. The difference isn't in technical skill—it's in time horizon and empathy.
The best senior developers I know write documentation not because they're required to, but because they've been the person trying to debug undocumented code at 3 AM too many times. They've felt the frustration of inheriting a system with no design rationale. They've made well-intentioned changes that broke things because they didn't understand the historical context.
They document not for recognition or process compliance, but for the simple human reason that they want to be kind to future maintainers—including themselves.
-ROHIT V.
Top comments (0)