Q515: How do you implement a singleton pattern in JavaScript? - JavaScript Internals (521 Qs)

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

Classic singleton with closure:

javascript

 const Singleton = (function() {
   let instance;

   function createInstance() {
     const object = {
       name: 'Singleton',
       data: []
     };
     return object;
   }

   return {
     getInstance() {
       if (!instance) {
         instance = createInstance();
       }
       return instance;
     }
   };
 })();

 const instance1 = Singleton.getInstance();
 const instance2 = Singleton.getInstance();
 console.log(instance1 === instance2); // true

ES6 class singleton:

javascript

 class Singleton {
   constructor() {
     if (Singleton.instance) {
       return Singleton.instance;
     }
     this.data = [];
     Singleton.instance = this;
   }

   addData(value) {
     this.data.push(value);
   }

   getData() {
     return this.data;
   }
 }

 const s1 = new Singleton();
 const s2 = new Singleton();
 console.log(s1 === s2); // true

Module singleton (simplest):

javascript

 // config.js
 class Config {
   constructor() {
     this.settings = {};
   }

   set(key, value) {
     this.settings[key] = value;
   }

   get(key) {
     return this.settings[key];
   }
 }

 export default new Config(); // Export single instance

 // usage.js
 import config from './config.js';
 config.set('apiUrl', 'https://api.example.com');

Singleton with WeakMap (private instance):

javascript

 const Singleton = (function() {
   const instances = new WeakMap();

   class Singleton {
     constructor(key) {
       if (instances.has(key)) {
         return instances.get(key);
       }
       this.data = [];
       instances.set(key, this);
     }
   }

   return Singleton;
 })();

Database connection singleton:

javascript

 class DatabaseConnection {
   constructor() {
     if (DatabaseConnection.instance) {
       return DatabaseConnection.instance;
     }

     this.connection = null;
     DatabaseConnection.instance = this;
   }

   connect(connectionString) {
     if (!this.connection) {
       this.connection = {
         connectionString,
         connected: true,
         queries: []
       };
     }
     return this.connection;
   }

   disconnect() {
     if (this.connection) {
       this.connection.connected = false;
       this.connection = null;
     }
   }

   query(sql) {
     if (this.connection?.connected) {
       this.connection.queries.push(sql);
       return `Executing: ${sql}`;
     }
     throw new Error('Not connected');
   }
 }

 const db1 = new DatabaseConnection();
 db1.connect('mongodb://localhost:27017');

 const db2 = new DatabaseConnection();
 console.log(db1 === db2); // true
 console.log(db2.query('SELECT * FROM users')); // Works