The Singleton pattern

JavaScript course · Expert · Lesson 2/63

Learn the Singleton pattern that ensures a single instance of a class.

The Singleton pattern

The Singleton pattern ensures that a class has only one instance. It is commonly used for configuration, caches or connections that must be shared.

In JavaScript we achieve this with a static instance property and a check inside the constructor. If the instance exists we return it; otherwise we create and store a new one.

This way the same configuration object is shared everywhere.

Singleton implementation

To implement a Singleton we start with static instance = null. The constructor returns AppConfig.instance if it is set; otherwise it assigns this to AppConfig.instance.

Often a static getInstance() method is added, so callers use it instead of new.

class AppConfig {
  static instance = null;
  constructor() {
    if (AppConfig.instance) return AppConfig.instance;
    AppConfig.instance = this;
  }
}

Exercise AppConfig singleton

Declare a Singleton class named AppConfig. Start with static instance = null. The constructor should return AppConfig.instance if it is set; otherwise it assigns this to AppConfig.instance.

Show example solution

Start with class AppConfig, add static instance = null, in the constructor do if (AppConfig.instance) return AppConfig.instance;, then assign AppConfig.instance = this.
Example solution:
class AppConfig {
  static instance = null;
  constructor() {
    if (AppConfig.instance) return AppConfig.instance;
    AppConfig.instance = this;
  }
}

Test yourself

What is the main goal of the Singleton pattern?
- Ensure only one instance of a class exists
- Sort objects
- Create many instances
- Hide the class
Show answer

Ensure only one instance of a class exists — Correct! Singleton guarantees a single instance.

Complete the Singleton assignment

class AppConfig {
  static instance = null;
  constructor() {
    if (AppConfig.instance) return AppConfig.instance;
    AppConfig.instance = ____;
  }
}

Show answer

class AppConfig {
  static instance = null;
  constructor() {
    if (AppConfig.instance) return AppConfig.instance;
    AppConfig.instance = this;
  }
}

Match the Singleton concepts

Singleton

Single instance guarantee
static instance

Class-level single instance
getInstance()

Instance access method
Lazy init

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The Singleton pattern