Cohesion, Coupling, and Separation of Concerns
Master three foundational software design properties that determine whether code stays maintainable: high cohesion, low coupling, and clear separation of concerns.
Cohesion, Coupling, and Separation of Concerns is a free Clean Architecture & Design Patterns in Practice lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Clean Architecture & Design Patterns in Practice learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Why These Properties Matter
Under clean code and SOLID lie three deeper design properties: cohesion, coupling, and separation of concerns. They explain why the rules work.
What is Cohesion
Cohesion measures how well a module’s responsibilities belong together. High cohesion = one well-defined job; low cohesion = a grab-bag of unrelated methods.
Low vs High Cohesion
Compare the two: the Util class mixes unrelated concerns, while UserRepository is tightly focused. That focus is high cohesion.
// Low cohesion
class Util {
saveUser() {}
sendEmail() {}
formatDate() {}
}
// High cohesion
class UserRepository {
save() {}
findById() {}
}What is Coupling
Coupling measures how much one module depends on another’s internals. Tight coupling spreads changes everywhere; loose coupling keeps interactions minimal.
Tight Coupling Example
Here OrderService creates its own SmtpMailer, locking it to that exact class. Swapping the mailer means editing the service — tight coupling.
class OrderService {
constructor() {
this.mailer = new SmtpMailer();
}
}Loosening the Coupling
Inject the mailer instead. Now OrderService depends on an abstraction and accepts any implementation. That’s loose coupling.
class OrderService {
constructor(mailer) {
this.mailer = mailer;
}
}The Sweet Spot
The sweet spot is high cohesion and low coupling together: focused modules that stay independent, so systems are easy to change, test, and reason about.
Separation of Concerns
Separation of concerns means splitting a program so each part owns one distinct concern — keep UI, business logic, and data access in separate layers.
Mixed Concerns
This function tangles data access, logic, and presentation together. Touch any one concern and you risk breaking the others — mixed concerns.
function showTotal(id) {
const rows = db.query('SELECT * FROM orders WHERE id=' + id);
const total = rows.reduce((s, r) => s + r.price, 0);
document.body.innerHTML = 'Total: ' + total;
}Separated Concerns
Split it into a repository, a calculator, and a view. Each concern now changes and tests independently. Much cleaner.
const order = repository.findById(id);
const total = calculator.total(order);
view.render(total);How They Connect to SOLID
These properties underpin SOLID: single responsibility drives cohesion, dependency inversion drives low coupling, and layering expresses separation of concerns.
Quick Check
High cohesion, low coupling — can you tell them apart?
Recap
You’ve got the foundations: cohesion keeps related work together, low coupling minimizes dependencies, and separation of concerns divides by purpose.
Frequently asked questions
Is the “Cohesion, Coupling, and Separation of Concerns” lesson free?
Yes — the full text of “Cohesion, Coupling, and Separation of Concerns” is free to read here on the web, and the Clean Architecture & Design Patterns in Practice course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Clean Architecture & Design Patterns in Practice course, upgrade to CoddyKit PRO.
What will I learn in “Cohesion, Coupling, and Separation of Concerns”?
Master three foundational software design properties that determine whether code stays maintainable: high cohesion, low coupling, and clear separation of concerns. You practise Clean Architecture & Design Patterns in Practice with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Clean Architecture & Design Patterns in Practice?
No prior experience is required. Clean Architecture & Design Patterns in Practice on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Cohesion, Coupling, and Separation of Concerns” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Clean Architecture & Design Patterns in Practice lesson?
Yes. Every Clean Architecture & Design Patterns in Practice lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Introduction to Clean Code
- Overview of SOLID Principles
- The Value of Good Design
- Cohesion, Coupling, and Separation of Concerns