What is a design pattern?

Learn what design patterns provide and common catalogs.

What is a design pattern?

A design pattern is a reusable solution template for a recurring problem in software design. You do not copy and paste the solution; you adapt the pattern to your own code.

Patterns are best practices shaped by experience. They give teams a shared vocabulary: when you say "use a Singleton here", everyone understands the intent.

A common catalog, known as "Gang of Four" (GoF), lists 23 patterns.

Pattern catalogs

The best-known catalog is the Gang of Four (GoF) book, which groups 23 patterns into three families:

Creational: organizes object creation. Examples: Singleton, Factory.
Structural: combines classes. Examples: Adapter, Decorator.
Behavioral: organizes communication. Examples: Observer, Strategy.

This group covers four common patterns: Singleton, Factory, Observer and Module.

Exercise Counter class

Declare a class named Counter. Its constructor sets this.count to 0. Its increment() method increases this.count by one and returns the new value.

Show example solution

Start with class, set this.count = 0 in the constructor, then in increment do this.count++ and return this.count.
Example solution:
class Counter {
  constructor() {
    this.count = 0;
  }
  increment() {
    this.count++;
    return this.count;
  }
}

Test yourself

How is a design pattern best described?
- A programming language feature
- A reusable solution template
- A debugging method
- A browser API
Show answer

A reusable solution template — Correct! A design pattern is a reusable solution template for a recurring problem.

Complete the console call

class Logger {
  log(message) {
    console.____(message);
  }
}

Show answer

class Logger {
  log(message) {
    console.log(message);
  }
}

Match the concepts

Design pattern

Reusable solution template
GoF catalog

Common pattern list
Creational

Object creation
Behavioral

Object interaction

Next lesson: The Singleton pattern

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