DRY (Don't Repeat Yourself)

DRY (Don't Repeat Yourself) is a software design principle, popularized by the book "The Pragmatic Programmer", stating that every piece of knowledge should have a single, unambiguous, authoritative representation within a system. It targets duplicated logic and facts, not merely similar-looking lines.

Why it matters

When the same rule is copied into several places, a change must be made everywhere, and missing one creates inconsistent behavior and bugs. Centralizing the rule in a function, constant, configuration value or schema makes changes safe and quick. Applied too eagerly, DRY produces premature abstractions: two pieces of code that look alike but change for different reasons get merged into a tangled shared helper full of flags. Many developers wait until a pattern appears three times before extracting it.

Example

Repeated:

const a = price * 1.18;   // tax in cart
const b = total * 1.18;   // tax in invoice

DRY version:

const TAX_RATE = 0.18;
const withTax = amount => amount * (1 + TAX_RATE);

Common mistakes

• Merging code that is only coincidentally similar.
• Creating a helper with many boolean parameters to cover every variation.
• Applying DRY across service boundaries, creating tight coupling.

Related

• Refactoring
• Technical debt
• Function
• SOLID principles

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