Observer & Singleton Patterns

What is the correct way to implement lazy initialization in a Singleton?

class LazySingleton {
  static #instance;
  #data;

  constructor() {
    if (LazySingleton.#instance) {
      return LazySingleton.#instance;
    }
    this.#data = null;
    LazySingleton.#instance = this;
  }

  static getInstance() {
    if (!LazySingleton.#instance) {
      LazySingleton.#instance = new LazySingleton();
    }
    return LazySingleton.#instance;
  }

  initialize() {
    if (this.#data === null) {
      this.#data = this.loadExpensiveData();
    }
    return this.#data;
  }

  loadExpensiveData() {
    // Expensive initialization logic
  }
}

Initialize data in the constructor
Create instance on script load
Initialize data only when first needed
This implementation demonstrates proper lazy initialization in a Singleton by: 1) Delaying expensive data initialization until needed, 2) Using private fields (#data) to protect internal state, 3) Checking for existing initialization before loading data, and 4) Providing a clear initialize method for when the data is actually needed. This approach improves application startup performance by deferring resource-intensive operations.
Use multiple initialization methods

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