Singleton

A singleton is a design pattern that restricts a class to a single instance and gives the rest of the program one well-known way to reach it. Typical uses are a configuration object, a logger or a connection manager.

Why it matters

Some resources should exist exactly once, and a singleton enforces that rule. It is also one of the most criticized patterns. Because it behaves like a global variable, it hides dependencies, makes code harder to test (state leaks between tests) and can cause trouble in multi-threaded programs if creation is not synchronized. Many teams instead create one instance at startup and pass it where needed, an approach supported by dependency injection.

Example

class Config {
  static #instance;
  static get() {
    return Config.#instance ??= new Config();
  }
}
console.log(Config.get() === Config.get()); // true

Both calls return the same object. In JavaScript, a module that exports one object is often an equivalent singleton.

Common mistakes

• Using a singleton as a convenient global for unrelated data.
• Lazily creating it in multi-threaded code without locking, producing two instances.
• Forgetting to reset its state between unit tests.

Related

• Design pattern
• Dependency injection
• Unit test
• Thread

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