Singleton Design Pattern

Introduction

• The singleton pattern is a type of creational pattern that restricts the instantiation of a class to a single object. This allows the class to create an instance of the class the first time it is instantiated. however, on the next try, the existing instance of the class is returned. No new instance is created.

• The singleton pattern is mostly used in cases where you want a single object to coordinate actions across a system.

• singleton pattern can be considered the basics of global state management libraries such as Redux or React Context.

• They can be accessed globally and acts as a single access point for accessing the global state

Use Cases

• Services: services can be singleton since they store the state, and configuration and provide access to resources; hence, it makes sense to have a single instance of a service in an application.

• Databases: when it comes to database connections, databases such as MongoDB utilize the singleton pattern.

• Configurations: if there is an object with a specific configuration, you don’t need a new instance every time that configuration object is needed.

Example

class Logger {
  constructor() {
    if (Logger.loggerInstance == null) {
      this.logs = [];
      Logger.loggerInstance = this;
    }
    return Logger.loggerInstance;
  }

  log(message) {
    this.logs.push(message);
    console.log(message);
  }

  printNumberOfLogs() {
    console.log("number of logs", ":", this.logs.length);
  }
}

const log1 = new Logger();
const log2 = new Logger();

log1.log("Error no 1");
log2.log("Error no 2");
console.log(log1.printNumberOfLogs(), log2.printNumberOfLogs());