How to Read and Understand Code You Didn’t Write

Imagine you’re stepping into a bustling kitchen, aprons flying, chefs bustling, each carrying out a recipe you’ve never seen before. You’re tasked with understanding — and eventually replicating — every dish coming out of the kitchen. This scenario mirrors the challenge of diving into a new codebase. It’s an overwhelming yet exciting endeavor, one that many developers find themselves facing repeatedly.

In the world of software development, reading and understanding code you didn’t write is not just a skill; it’s a necessity. While countless courses teach you how to write code from scratch, few prepare you for the reality of deciphering existing codebases. Yet, it’s this ability that frequently separates effective developers from the rest.

Without mastering this skill, you risk stalling your projects, misinterpreting functionality, and potentially derailing entire systems. But fear not — by honing your code-reading skills, you can unlock the full potential of any project you encounter, building on what exists with confidence and precision.

The Skill Nobody Teaches

Programming education is almost entirely focused on writing code. How to structure logic, how to use language features, how to build systems from scratch. What gets comparatively little attention is reading code — understanding codebases that someone else wrote, in a state that may be partially documented, under time pressure, on projects you didn’t design.

Experienced developers read more code than they write — often at a ratio of 10:1.

Studies of professional software development consistently find that programmers read far more code than they write. New hires, for instance, spend months reading existing codebases before they can contribute confidently. At Google, new engineers often dedicate the first three months to understanding the sprawling codebase before they can meaningfully contribute. This highlights the critical importance of reading skills.

Understanding the existing code is not just about delving deep into individual lines but about appreciating the logic and architecture laid by predecessors. It’s akin to understanding the blueprint before deciding where to place the furniture.

Start With the Entry Points

The first step to decoding an unfamiliar codebase is identifying its entry points. Every application, whether a web server or a mobile app, has a starting point where execution begins. For instance, in a Node.js application, the server.js file often serves as the main entry point. By locating these critical junctures, you gain orientation within the vast map of code.

Take Airbnb’s codebase as an example. New developers focus initially on entry points within their React Native app to understand navigation flows before diving into complex components. This strategy helps them grasp the application’s primary functionality without getting lost in the minutiae.

Resist the temptation to read a codebase top-to-bottom, file by file. That approach produces a lot of context with no organizing structure. Start where execution starts, follow what happens, and expand outward from there. It’s like reading the first chapter of a book to understand the plot before examining the characters’ backstories.

Read for Intent Before Reading for Mechanics

When you encounter a function you don’t understand, try to figure out its intent before you understand its implementation. The function name, its inputs, its outputs, and its position in the call stack tell you a lot about what it’s supposed to do. Understanding intent first prevents a common failure mode: spending thirty minutes understanding the mechanics of a function before realizing it’s a utility that handles an edge case you don’t care about.

For instance, consider a function in a large e-commerce platform like Shopify. If you’re tasked with understanding a discount calculation module, identify what it’s supposed to achieve first. Is it applying seasonal discounts, or is it adjusting prices based on customer segments? Knowing this will help you decide how much depth you need to explore within the implementation.

Use the Tools Available to You

Modern IDEs have dramatically reduced the friction of reading unfamiliar code. “Go to definition” lets you follow function calls to their source. “Find all references” shows you everywhere a function or variable is used. “Git blame” shows who wrote each line and when — often providing context for why something is done the way it is. These tools are obvious once you know about them, but many beginners don’t use them systematically.

For particularly complex code, a debugger is often better than reading. Setting a breakpoint and stepping through execution in real time, with actual values visible, resolves confusion that static reading leaves open. You’re no longer reasoning about what the code does — you’re watching it do it. For instance, Visual Studio Code’s debugging tools allow you to step through JavaScript applications line by line, observing variable states in real-time.

Notes and Diagrams Help More Than People Think

When navigating a complex codebase for the first time, keeping notes helps significantly — not because you’ll reference them later but because the act of writing what you understand forces more careful reading. A rough diagram of how components call each other, a list of the main data structures and what they represent, a glossary of domain-specific terms — these take thirty minutes to build and save hours of re-reading the same code multiple times.

Consider engineers at Facebook who often use extensive documentation and visual aids to navigate their massive codebases. These diagrams not only help in understanding but also in communicating complex systems to new team members. It’s a practice that turns mental maps into tangible guides.

Ask Questions Strategically

When joining a new codebase with a team, ask about the architecture before asking about specific implementation choices. Understanding the big-picture decisions — why this database, why this framework, why this service split — makes specific implementation choices much more legible. Save your specific questions for when you’ve already invested serious reading time.

For example, at companies like Stripe, engineers are encouraged to ask pointed questions that reflect their understanding of the codebase. Instead of asking broad questions like, “How does the payment system work?” they would say, “I’ve gone through the transaction handling code, but I’m unclear about how we handle currency conversion.” This approach maximizes learning and minimizes disruption.

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Sources

• Martin, R. C. (2008). Clean Code. Prentice Hall.
• Fowler, M. (1999). Refactoring: Improving the Design of Existing Code. Addison-Wesley.
• Spinellis, D. (2003). Code Reading: The Open Source Perspective. Addison-Wesley.