Creating the Lazy pattern construct in javascript

README.md

when fetching an expensive and costly object that might not be used frequently or not at all, its best to load and fetch it lazily, ie, only fetch it when the process requires it.

the below code is an implementation of the Lazy pattern which is already available in C#.

export default class Lazy {
  #value;
  #fetch;

  constructor(valueFetcher) {
    this.#fetch = valueFetcher;
  }

  value = () => this.#value || (this.#value = this.#fetch());
}

Tests:

import Lazy from '../lazy';

describe('Lazy', () => {
  it('only fetches value when asked for it', () => {
    let wasFetched = false;
    const item = new Lazy(() => {
      wasFetched = true;
      return 1;
    });

    expect(wasFetched).toBe(false);
    expect(item.value()).toBe(1);
  });

  it('returns value when asked for', () => {
    let wasFetched = false;
    const item = new Lazy(() => {
      wasFetched = true;
      return 1;
    });

    expect(item.value()).toBe(1);
    expect(wasFetched).toBe(true);
  });

  it('returns saved value when asked for it multiple times', () => {
    let fetchCount = 0;
    const item = new Lazy(() => {
      fetchCount++;
      return 1;
    });

    expect(item.value()).toBe(1);
    expect(item.value()).toBe(1);
    expect(fetchCount).toBe(1);
  });

  it('updates saved value when asked to do so', () => {
    const item = new Lazy(() => ({innerValue : 1}));
    item.value().innerValue = 2;
    expect(item.value().innerValue).toBe(2);
  });
});