The DRY Principle

Nov 19, 2025 - 2 min read - 500 words

Definition

Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.

~ in The Pragmatic Programmer by Andrew Hunt and David Thomas

The concept

DRY stands for Don’t Repeat Yourself. It is a fundamental principle of software development that aims to reduce repetition.

When you duplicate code, you create a maintenance burden. If the logic needs to change, you have to update it in multiple places. If you miss one, you introduce a bug. By keeping your code DRY, you ensure that a change in logic only needs to be made in one place.

However, DRY isn’t just about code duplication. It’s about knowledge duplication. If two pieces of code look different but represent the same business rule, they violate DRY. Conversely, if two pieces of code look identical but represent different concepts that just happen to coincide, combining them might be a mistake (often called “WET” - Write Everything Twice, or We Enjoy Typing).

The Rule of Three

A common rule of thumb is the Rule of Three:

“Two instances of similar code is just a coincidence, but three is a pattern.”

It is often better to tolerate a little duplication than to create the wrong abstraction too early.

Pitfalls: Incidental vs. Essential Duplication

Be careful not to confuse essential duplication (same business logic) with incidental duplication (code that just happens to look the same).

If you combine two pieces of code that look similar but serve different business needs, you couple them. When one business need changes, you’ll break the other. This is often worse than duplication.

Examples

In JavaScript

Refactoring repetitive logic into reusable functions.

Bad

const user1 = { firstName: 'John', lastName: 'Doe' };
console.log(`Hello, ${user1.firstName} ${user1.lastName}!`);

const user2 = { firstName: 'Jane', lastName: 'Smith' };
console.log(`Hello, ${user2.firstName} ${user2.lastName}!`);

Good

const getFullName = (user) => `${user.firstName} ${user.lastName}`;
const greetUser = (user) => console.log(`Hello, ${getFullName(user)}!`);

const user1 = { firstName: 'John', lastName: 'Doe' };
const user2 = { firstName: 'Jane', lastName: 'Smith' };

greetUser(user1);
greetUser(user2);

In CSS

Instead of repeating the same color values throughout your stylesheets, use CSS variables (custom properties).

Bad

.button-primary {
  background-color: #007bff;
}

.link {
  color: #007bff;
}

.card {
  border: 1px solid #007bff;
}

Good

:root {
  --primary-color: #007bff;
}

.button-primary {
  background-color: var(--primary-color);
}

.link {
  color: var(--primary-color);
}

.card {
  border: 1px solid var(--primary-color);
}

A Note on Testing

In testing, the DRY principle is often relaxed in favor of DAMP (Descriptive And Meaningful Phrases). Readable tests that clearly state their setup and expectations are often better than highly abstracted tests that are hard to debug, even if they contain some repetition.

Resources

The Pragmatic Programmer Wikipedia
Don’t Repeat Yourself Wikipedia