How to define Singleton in TypeScript

Question

What is the best and most convenient way to implement a Singleton pattern for a class in TypeScript? (Both with and without lazy initialisation).

Answer 1

This is the simplest way

class YourSingletoneClass {
  private static instance: YourSingletoneClass;

  private constructor(public ifYouHaveAnyParams: string) {

  }
  static getInstance() {
    if(!YourSingletoneClass.instance) {
      YourSingletoneClass.instance = new YourSingletoneClass('If you have any params');
    }
    return YourSingletoneClass.instance;
  }
}

Answer 2

After scouring this thread and playing around with all the options above - I settled with a Singleton that can be created with proper constructors:

export default class Singleton {
  private static _instance: Singleton

  public static get instance(): Singleton {
    return Singleton._instance
  }

  constructor(...args: string[]) {
    // Initial setup

    Singleton._instance = this
  }

  work() { /* example */ }

}

It would require an initial setup (in main.ts, or index.ts), which can easily be implemented by
new Singleton(/* PARAMS */)

Then, anywhere in your code, just call Singleton.instnace; in this case, to get work done, I would call Singleton.instance.work()

Answer 3

I have found a new version of this that the Typescript compiler is totally okay with, and I think is better because it doesn't require calling a getInstance() method constantly.

import express, { Application } from 'express';

export class Singleton {
  // Define your props here
  private _express: Application = express();
  private static _instance: Singleton;

  constructor() {
    if (Singleton._instance) {
      return Singleton._instance;
    }

    // You don't have an instance, so continue

    // Remember, to set the _instance property
    Singleton._instance = this;
  }
}

This does come with a different drawback. If your Singleton does have any properties, then the Typescript compiler will throw a fit unless you initialize them with a value. That's why I included an _express property in my example class because unless you initialize it with a value, even if you assign it later in the constructor, Typescript will think it hasn't been defined. This could be fixed by disabling strict mode, but I prefer not to if possible. There is also another downside to this method I should point out, because the constructor is actually getting called, each time it does another instance is technically created, but not accessible. This could, in theory, cause memory leaks.

Answer 4

Another option is to use Symbols in your module. This way you can protect your class, also if the final user of your API is using normal Javascript:

let _instance = Symbol();
export default class Singleton {

    constructor(singletonToken) {
        if (singletonToken !== _instance) {
            throw new Error("Cannot instantiate directly.");
        }
        //Init your class
    }

    static get instance() {
        return this[_instance] || (this[_instance] = new Singleton(_singleton))
    }

    public myMethod():string {
        return "foo";
    }
}

Usage:

var str:string = Singleton.instance.myFoo();

If the user is using your compiled API js file, also will get an error if he try to instantiate manually your class:

// PLAIN JAVASCRIPT: 
var instance = new Singleton(); //Error the argument singletonToken !== _instance symbol

Answer 5

Singleton classes in TypeScript are generally an anti-pattern. You can simply use namespaces instead.

Useless singleton pattern

class Singleton {
    /* ... lots of singleton logic ... */
    public someMethod() { ... }
}

// Using
var x = Singleton.getInstance();
x.someMethod();

Namespace equivalent

export namespace Singleton {
    export function someMethod() { ... }
}
// Usage
import { SingletonInstance } from "path/to/Singleton";

SingletonInstance.someMethod();
var x = SingletonInstance; // If you need to alias it for some reason

Answer 6

The following approach creates a Singleton class that can be used exacly like a conventional class:

class Singleton {
    private static instance: Singleton;
    //Assign "new Singleton()" here to avoid lazy initialisation

    constructor() {
        if (Singleton.instance) {
            return Singleton.instance;
        }

        this. member = 0;
        Singleton.instance = this;
    }

    member: number;
}

Each new Singleton() operation will return the same instance. This can however be unexpected by the user.

The following example is more transparent to the user but requires a different usage:

class Singleton {
    private static instance: Singleton;
    //Assign "new Singleton()" here to avoid lazy initialisation

    constructor() {
        if (Singleton.instance) {
            throw new Error("Error - use Singleton.getInstance()");
        }
        this.member = 0;
    }

    static getInstance(): Singleton {
        Singleton.instance = Singleton.instance || new Singleton();
        return Singleton.instance;
    }

    member: number;
}

Usage: var obj = Singleton.getInstance();