Singleton

A simple singleton implementation with lazy initialization.

Installation

Simply run:

npm install @heap-code/singleton

CDN

Thanks to jsdelivr, this package can easily be used in browsers like this:

<script
 src="https://cdn.jsdelivr.net/npm/@heap-code/singleton/dist/bundles/singleton.umd.js"
 type="application/javascript"
></script>

Note:
It is recommended to use a minified and versioned bundle.

For example:

<script
 src="https://cdn.jsdelivr.net/npm/@heap-code/[email protected]/dist/bundles/singleton.umd.min.js"
 type="application/javascript"
></script>

More at this jsdelivr package page.

Usage

Wrap a value that should only be calculated once and only when needed:

// Typescript
import { Singleton } from "@heap-code/singleton";

const mySingleton = new Singleton(() => Math.random());
console.log(mySingleton.get() === mySingleton.get());  // true

Promise

It also works with Promise as they are only fulfilled once:

import { Singleton } from "@heap-code/singleton";

function doAsyncStuff() {
  return new Promise(resolve => {
    console.log("It doesn't look like it, but I'm actually doing a lot of things.");
    setTimeout(() => resolve(new Date().getMilliseconds()), 100);
  });
}

async function bootstrap() {
  const a1 = await doAsyncStuff();
  const a2 = await doAsyncStuff();
  console.log(a1 === a2); // false

  const singleton = new Singleton(() => doAsyncStuff());
  const b1 = await singleton.get();
  const b2 = await singleton.get();
  console.log(b1 === b2); // true
}

bootstrap();

Note:
Rather use this library for its lazy initialization rather than its "singletoness":

Example:

import { Singleton } from "@heap-code/singleton";

class MyAddition {
  // Simply calculated, often used
  public readonly added: number;

  private readonly singleton: Singleton<number>;

  public constructor(private readonly a: number, private readonly b: number) {
    this.added = a + b;
    this.singleton = new Singleton(() => Math.pow(a * b, Math.sin(a) * Math.cos(b)));
  }

  public get divided() {
    // Used sometimes
    return this.a / this.b;
  }

  public get complicated() {
    // Used only sometimes and performance-intensive.
    // Calculated only once needed
    return this.singleton.get();
  }
}

Releases

See information about breaking changes and release notes here.

Author's note

This is not the most useful package, as it can most of the time be simply replaced by a variable. And come consider it to be an anti-pattern (Singleton pattern criticism).

This package was more a test for automating changelogs generation, GitHub and npm publishing process.