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Singleton Manager: Overview

A singleton manager provides a way to make sure a singleton instance loaded from multiple file locations stays a singleton. Primarily useful if two major version of a package with a singleton is used.


npm i --save singleton-manager

⚠️ You need to make SURE that only ONE version of singleton-manager is installed. For how see Non Goals.

Example Singleton Users

Use the same singleton for both versions (as we don't use any of the breaking features)

// managed-my-singleton.js
import { singletonManager } from 'singleton-manager';
import { mySingleton } from 'my-singleton'; // is available as 1.x and 2.x via node resolution

singletonManager.set('my-singleton::index.js::1.x', mySingleton);
singletonManager.set('my-singleton::index.js::2.x', mySingleton);

OR create a special compatible version of the singleton

// managed-my-singleton.js
import { singletonManager } from 'singleton-manager';
import { MySingleton } from 'my-singleton'; // is available as 1.x and 2.x via node resolution

class CompatibleSingleton extends MySingleton {
  // add forward or backward compatibility code
const compatibleSingleton = new CompatibleSingleton();

singletonManager.set('my-singleton::index.js::1.x', compatibleSingleton);
singletonManager.set('my-singleton::index.js::2.x', compatibleSingleton);

AND in you App then you need to load the above code BEFORE loading the singleton or any feature using it.

import './managed-my-singleton.js';

import { mySingleton } from 'my-singleton'; // will no always be what is "defined" in managed-my-singleton.js


Overriding version is an App level concern hence components or "features" are not allowed to use it. If you try to call it multiple times for the same key then it will be ignored.

// on app level
singletonManager.set('my-singleton/index.js::1.x', compatibleSingleton);

// somewhere in a dependency
singletonManager.set('my-singleton/index.js::1.x', otherSingleton);

// .get('my-singleton/index.js::1.x') will always return the first set value
// e.g. the app can set it and no one can later override it

Example Singleton Maintainers

If you are a maintainer of a singleton be sure to check if a singleton manager version is set. If that is the case return it instead of your default instance.

It could look something like this:

// my-singleton.js
import { singletonManager } from 'singleton-manager';
import { MySingleton } from './src/MySingleton.js';

export const overlays =
  singletonManager.get('my-singleton/my-singleton.js::1.x') || new MySingleton();

Convention Singleton Key

The key for a singleton needs to be "unique" for the package. Hence the following convention helps maintaining this.

As a key use the <package>::<unique-variable>::<semver-range>.

Examples Do:

  • overlays::overlays::1.x - instance created in index.js
  • @scope/overlays::overlays::1.x - with scope
  • overlays::overlays::1.x - version 1.x.x (> 1.0.0 you do 1.x, 2.x)
  • overlays::overlays::2.x - version 2.x.x (> 1.0.0 you do 1.x, 2.x)
  • overlays::overlays::0.10.x - version 0.10.x (< 1.0.0 you do 0.1.x, 0.2.x)

Examples Don't:

  • overlays - too generic
  • overlays::overlays - you should include a version
  • overlays::1.x - you should include a package name & unique var
  • ./index.js::1.x - it should start with a package name

Singleton Manager Rationale

We have an app with 2 pages.

  • page-a uses overlays 1.x
  • page-b uses overlays 2.x (gets installed nested)
my-app (node_modules)
├── overlays (1.x)
├── page-a
│ └── page-a.js
└── page-b
├── node_modules
│ └── overlays (2.x)
└── page-b.js

The tough part in this case is the OverlaysManager within the overlays package as it needs to be a singleton.

It starts of simplified like this

export class OverlaysManager {
  name = 'OverlayManager 1.x';
  blockBody = false;
  constructor() {
  _setupBlocker() {
    /* ... */
  block() {
    this.blockBody = true; // ...

  unBlock() {
    this.blockBody = false; // ...

Example A (fail)

See it "fail" e.g. 2 separate OverlaysManager are at work and are "fighting" over the way to block the body.

npm run start:fail

Steps to reproduce:

  1. Page A click on block
  2. Page B => "Blocked: false" (even when hitting the refresh button)

➡️ See it on the example page.
➡️ See the code.

Example B (singleton manager)

The breaking change in OverlayManager was renaming of 2 function (which has been deprecated before).

  • block() => blockingBody()
  • unBlock() => unBlockingBody()

knowing that we can create a Manager that is compatible with both via

import { OverlaysManager } from 'overlays';

class CompatibleOverlaysManager extends OverlaysManager {
  blockingBody() {
  unBlockingBody() {

all that is left is a to "override" the default instance of the "users"

import { singletonManager } from 'singleton-manager';

const compatibleOverlaysManager = new CompatibleOverlaysManager();
singletonManager.set('overlays::overlays::1.x', compatibleOverlaysManager);
singletonManager.set('overlays::overlays::2.x', compatibleOverlaysManager);

See it in action

npm run start:singleton

➡️ See it on the example page.
➡️ See the code.

Example C (singleton and complex patching on app level)

The breaking change in OverlayManager was converting a property to a function and a rename of a function.

  • blockBody => _blockBody
  • block() => blockBody()
  • unBlock() => unBlockBody()

e.g. what is impossible to make compatible with a single instance is to have blockBody act as a property for 1.x and as a function blockBody() for 2.x.

So how do we solve it then?

We will make 2 separate instances of the OverlayManager.

compatibleManager1 = new CompatibleManager1(); // 1.x
compatibleManager2 = new CompatibleManager2(); // 2.x
console.log(typeof compatibleManager1.blockBody); // Boolean
console.log(typeof compatibleManager2.blockBody); // Function

// and override
singletonManager.set('overlays::overlays::1.x', compatibleManager1);
singletonManager.set('overlays::overlays::2.x', compatibleManager2);

and they are "compatible" to each other because they sync the important data to each other. e.g. even though there are 2 instances there is only one dom element inserted which both can write to. When syncing data only the initiator will update the dom. This makes sure even though functions and data is separate it will be always consistent.

See it in action

npm run start:singleton-complex

➡️ See it on the example page.
➡️ See the code.

How does it work?

As a user you can override what the import of overlays/instance.js provides. You do this via a singletonManager and a "magic" string.

  • Reason be that you can target ranges of versions
singletonManager.set('overlays::overlays::1.x', compatibleManager1);
singletonManager.set('overlays::overlays::2.x', compatibleManager2);

Potential Improvements

Potentially we could have "range", "exacts version" and symbol for unique filename. So you can override with increasing specificity. If you have a use case for that please open an issue.

Non Goals

Making sure that there are only 2 major versions of a specific packages. npm is not meant to handle it - and it never will

├─┬ feat-a@x
│ └── foo@2.x
├─┬ feat-a@x
│ └── foo@2.x
└── foo@1.x

Dedupe works by moving dependencies up the tree

// this app

// can become if versions match

in there feat-a will grab the version of it's "parent" because of the node resolution system. If however the versions do not match or there is no "common" folder to move it up to then it needs to be "duplicated" by npm/yarn.

Only by using a more controlled way like

you can "hard" code it to the same versions.