All 10 Design Patterns in JavaScript questions
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What is the Module pattern and what problem does it solve? How do you implement the Observer pattern in JavaScript? What is the Factory pattern, and how does it differ from calling a class constructor directly? How is the Decorator pattern typically implemented in JavaScript? What is the Strategy pattern and how does it help avoid large if/else chains? How do you implement the Proxy pattern using JavaScript's built-in Proxy object? What is the Command pattern and where is it useful in JavaScript apps? What is the Adapter pattern and why is it useful when integrating third-party code? When is it a mistake to force a 'classic' design pattern into idiomatic JavaScript?
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What is the Singleton pattern and how is it implemented in JavaScript?

Beginner

Answer

Singleton ensures only one instance of an object exists across the whole application, typically by exporting a single already-created instance from a module — since ES modules are cached, importing it anywhere returns the same instance.
// config.js
class Config {
  constructor() { this.settings = {}; }
}
export default new Config(); // same instance everywhere it's imported
Real-world example A single shared application configuration or logger instance used across many files.

Common follow-ups: What are the downsides of overusing Singletons, especially for testing?

ES Modules
All 10 Design Patterns in JavaScript questions