Building Mental Models: How to Actually Understand Code by Reading It

Here’s something they don’t teach you in most programming courses: reading code is a completely different skill from writing code. And honestly? It might be more important.

Think about it. In your career, you’ll spend way more time reading existing code than writing new stuff from scratch. You’ll be debugging someone else’s implementation, adding features to legacy systems, or trying to understand how that critical service actually works. Yet most of us learn programming by writing, not reading.

What’s a Mental Model, Anyway?

A mental model is basically your brain’s simplified version of how something works. When you think about a car, you don’t picture every bolt and wire—you have a mental model that includes stuff like “engine makes power, wheels turn, steering wheel controls direction.”

For code, mental models help you understand the big picture without getting lost in every semicolon and variable name. Instead of trying to memorize every line, you build frameworks for understanding what the code is actually doing and why.

The Problem with How Most People Read Code

Most beginners (and plenty of experienced developers) read code like they’re reading a novel—line by line, from top to bottom, trying to understand every detail before moving on. This is exhausting and usually doesn’t work.

Here’s what happens: you get bogged down in implementation details, lose track of the bigger picture, and end up more confused than when you started. You might understand what each line does, but you have no clue why the code exists or how it fits together.

It’s like trying to understand a city by reading the phone book. Sure, you’ll learn some street addresses, but you won’t know how to get around.

The Better Way: Think in Layers

Professional developers read code in layers, building up their understanding gradually. Here’s how to do it:

Layer 1: What Does This Thing Do?

Before diving into any code, figure out its purpose. Look for:

• Function/class names - They usually tell you the main job
• Comments at the top - Often explain the why, not just the how
• Input and output - What goes in, what comes out
• File/module structure - How is functionality organized?

Don’t worry about how it works yet. Just get the 30,000-foot view.

Layer 2: What Are the Main Parts?

Now identify the key components and their relationships:

• Main functions or classes - The heavy lifters
• Data structures - How information is organized
• Control flow - The general path through the code
• Dependencies - What external stuff does this rely on?

You’re building a rough map of the territory before exploring the details.

Layer 3: How Do the Parts Work?

Finally, dive into implementation details—but only after you understand the context:

• Algorithms and logic - The step-by-step process
• Error handling - What happens when things go wrong
• Edge cases - Special situations the code handles
• Performance considerations - Why certain approaches were chosen

Practical Mental Models for Common Code Patterns

Here are some mental models that work for most code you’ll encounter:

The Pipeline Model

Think of data flowing through a series of transformations, like an assembly line. Good for understanding APIs, data processing, and most web applications.

Example: User request → validation → database query → format response → send back

The State Machine Model

Think of the program as having different modes or states, with rules about how to transition between them. Great for user interfaces, game logic, and workflow systems.

Example: Login screen → dashboard → specific feature → logout

The Layer Cake Model

Think of functionality stacked in layers, where each layer only talks to the layers immediately above or below it. Perfect for understanding frameworks, operating systems, and well-architected applications.

Example: Database → API layer → business logic → user interface

The Event-Driven Model

Think of the program as responding to things that happen, rather than following a predetermined path. Essential for understanding modern web applications, games, and real-time systems.

Example: User clicks button → event fires → handler executes → UI updates

How to Build Your Own Mental Models

Every codebase is different, so you’ll need to develop your own mental models. Here’s how:

1. Start with Questions, Not Answers

Before reading any code, ask yourself:

• What problem is this solving?
• Who are the users?
• What are the main use cases?
• What could go wrong?

These questions guide your reading and help you focus on what matters.

2. Draw Pictures (Seriously)

Grab a piece of paper or open a drawing tool and sketch out your understanding. It doesn’t have to be pretty—boxes and arrows work fine. The act of drawing forces you to be explicit about relationships and reveals gaps in your understanding.

3. Trace Through Scenarios

Pick a typical use case and follow it through the code from start to finish. Don’t get distracted by edge cases or error handling on your first pass. Just follow the happy path and see how the pieces connect.

4. Look for Patterns You Recognize

Most code follows common patterns. When you see something that looks familiar, start with that assumption and see if it fits. MVC architecture, observer pattern, factory methods—if you know these patterns, use them as starting points for understanding new code.

5. Question Your Assumptions

As you build your mental model, actively look for evidence that contradicts it. Good mental models evolve as you learn more. If something doesn’t fit your current understanding, that’s interesting—dig deeper.

Real-World Example: Reading a Web API

Let’s say you’re looking at a REST API for a todo app. Here’s how you’d apply mental models:

Layer 1: This is a web service that manages todo items. Users can create, read, update, and delete their tasks.

Layer 2: There’s probably a database for persistence, some kind of web framework for handling HTTP requests, and business logic for managing todos. The main endpoints likely map to CRUD operations.

Layer 3: Now you look at specific implementations—how are todos stored, what validation happens, how are errors handled, what about authentication?

Mental Model: Pipeline model works well here. HTTP request → routing → authentication → business logic → database → format response → HTTP response.

Why This Approach Changes Everything

When you read code with mental models, several things happen:

You understand faster. Instead of getting lost in details, you build understanding systematically from the outside in.

You retain more. Mental models give you a framework to hang details on. Instead of remembering random facts, you remember how things relate to each other.

You can make changes confidently. When you understand the mental model, you know where to look when you need to modify behavior. You can predict what will break and what will keep working.

You become a better debugger. Bugs often happen when reality doesn’t match the mental model. When something’s not working, you can systematically check each layer of your understanding.

The Long Game

Building good mental models takes practice, but it’s one of the most valuable skills you can develop. It’s the difference between being someone who can follow tutorials and being someone who can understand and modify any codebase they encounter.

Start with small, well-documented projects. Practice identifying patterns. Draw diagrams. Ask questions. Most importantly, don’t try to understand everything at once—build your understanding in layers.

Six months from now, you’ll find yourself looking at complex codebases and thinking “Oh, I see what’s happening here” instead of “What the hell is all this?” That’s the power of mental models in action.

Trust me, this skill will serve you for your entire career. The technology changes, but the ability to quickly understand how systems work? That’s timeless.