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Singleton Pattern

Singleton Pattern

Introduction

The Singleton pattern ensures a class has only one instance and provides a global point of access to it.

Topics Covered

Implementation — Ensuring a single instance
Pros & Cons — When to use (and avoid) singletons

Implementation

class Database {
  constructor() {
    if (Database.instance) {
      return Database.instance;
    }
    this.connection = null;
    Database.instance = this;
  }

  connect(url) {
    this.connection = url;
    console.log(`Connected to ${url}`);
  }

  query(sql) {
    if (!this.connection) throw new Error('Not connected');
    return `Result of: ${sql}`;
  }
}

const db1 = new Database();
const db2 = new Database();
console.log(db1 === db2); // true

Summary

Single instance guarantee
Often controversial — can make testing difficult
Useful for configuration, logging, database connections

Further Reading

Refactoring Guru: Singleton