How to Read Other Peoples Code
Senior developers spend more time reading code than writing it. Here is the skill that nobody teaches.
The Skill Schools Do Not Teach
Every CS curriculum and every bootcamp focuses on writing code. Almost none focus on reading it. That is strange because in a real job you will read 10x more code than you write - most of it written by someone else, often years ago, often without comments.
The good news: reading is a learnable skill, not a talent. The bad news: most beginners try to read unfamiliar code top to bottom, every line, and get hopelessly lost.
Technique 1: Start From the Entry Point
Every program has an entry point. main() in Java/C. Bottom of the file in Python. app.listen() in Node. createRoot() in modern React.
Find it. From there, follow the function calls in execution order. Do not read alphabetically.
Technique 2: Read the Tests First
If the codebase has tests, read them before reading the code. Tests document intent: this function should return 0 when given an empty array. That is a complete spec without reading the implementation.
Technique 3: Trace One Feature End-to-End
Pick one feature - say user clicks Login. Follow it through every layer: click handler, API call, route, controller, database query, response, redirect. By the end you understand the architecture.
Technique 4: Use the Debugger to Read
Set a breakpoint at the entry point. Step through line by line. Watch what variables hold what values. Execution becomes a tour guide.
Available in Chrome DevTools, every IDE, and pdb for Python.
Technique 5: Take Notes In Your Own Words
Open a scratch file. As you read, write a one-sentence description of every function in your own words. After a few hours you have a mental map.
Anti-Patterns to Avoid
- Reading every file alphabetically. File ordering reflects the file system, not the architecture.
- Trying to understand everything before writing anything. Make a small change after the first day.
- Refactoring on first encounter. Wait three weeks. The ugliness usually has a reason.
- Asking the team after 3 minutes. Try for 30 minutes first. Not 3 days, not 3 minutes.
How to Practice This Today
Pick a small open-source project you use. Clone it. Try the techniques above. Suggested for beginners: Express.js, Lodash.
For the broader how to learn arc, see how long does it take to learn coding.
Git History as Documentation
Most beginners do not realize git history is a first-class debugging tool. git log -p path/to/file shows every change that file has ever received, with the commit message explaining why. git blame tells you who last changed a particular line and when. Both are gold for understanding "why does this look weird?"
The pattern: when you find code that does not make sense, run git blame. The commit message often explains the bug it was fixing. The Pro Git book chapter on debugging with git covers this beautifully.
Reverse-Engineering an Architecture
For larger codebases, finding the architecture is harder than finding any specific function. Three habits:
- Look for the configuration files.
package.json,requirements.txt,pom.xml, plus framework configs - they tell you what libraries are in play and roughly how the app boots. - Find the routing layer. Every web app has a place where URLs map to handlers. That layer is the table of contents.
- Identify the data model. SQL schemas, ORM models, or TypeScript interfaces tell you what the application thinks the world contains. Once you know the nouns, the verbs (functions) make sense.
Using AI to Read Code
AI assistants are surprisingly good at "summarize what this file does in three bullets." For unfamiliar codebases, this can save real hours - but with the same caveat as always: verify. AI confidently misunderstands code, especially around subtle behavior.
Best practice: ask AI for a summary, then verify by reading the function it summarized. The summary is a hypothesis to confirm, not a fact.
Practice Resources
Small open-source codebases worth reading as exercises:
- sindresorhus/is - tiny utility library, clean code.
- Lodash - bigger, but each file is small and focused.
- Express.js - core web framework, surprisingly readable.
For broader skill-building, see git: the five commands that matter and naming things.
Frequently Asked Questions
How long should I spend reading before writing?
For a feature, 30 minutes of reading before any change. For a bug, however long it takes to reproduce. For a whole codebase, never finish reading first - touch something small in week one.
Should I take notes when reading code?
Yes. A scratch file with one-sentence summaries of every function you encounter pays back tenfold. After a week the file becomes a search-able mental map.
How do I handle reading code in a language I do not know well?
Lean harder on tests, types, and comments. Use the language's docs alongside reading. Resist the urge to translate every keyword - patterns transfer faster than syntax memorization.
What if the codebase has no tests or comments?
Use git blame to find when each suspicious bit was added; the commit messages may explain. Trace one feature end to end as your tour. Add a few tests yourself as you learn - that is the most valuable contribution you can make.
Key Takeaways
- Reading code is a separate skill from writing it. Most jobs require it more.
- Start at the entry point, not file 1 alphabetically. Trace one feature end-to-end before reading widely.
- Read tests before reading implementations - they document intent more clearly.
- Use a debugger or AI summary as your tour guide, not a replacement for understanding.
- Take notes in your own words. The act of summarizing forces comprehension.
- Make a small change in week one. Reading without doing leads to false confidence.