Make a class Singleton using decorator (via Proxy). Enable decorators by setting "experimentalDecorators": true in your tsconfig.json.

singleton.test.ts

import { describe, expect, it } from "bun:test"
import { Singleton } from "./singleton"

describe("@Singleton", () => {
  it("makes class singleton", () => {
    @Singleton
    class Foo {
      constructor(public bar: string) {}
    }

    class Bar {}
    const SingletonBar = Singleton(Bar)

    const foo1 = new Foo("bar")
    const foo2 = new Foo("??????")
    const bar1 = new SingletonBar()
    const bar2 = new SingletonBar()

    expect(foo2).toBe(foo1)
    expect(bar2).toBe(bar1)
    expect(foo1.bar).toBe("bar")
    expect(foo2.bar).toBe("bar")
  })
})

singleton.ts

export const SINGLETON_KEY = Symbol("Singleton Key")

export type Singleton<T extends new (...args: any[]) => any> = T & {
  [SINGLETON_KEY]: T extends new (...args: any[]) => infer I ? I : never
}

export function Singleton<T extends new (...args: any[]) => any>(
  classTarget: T
) {
  return new Proxy(classTarget, {
    construct(target: Singleton<T>, argumentsList, newTarget) {
      if (target.prototype !== newTarget.prototype) {
        return Reflect.construct(target, argumentsList, newTarget)
      }

      if (!target[SINGLETON_KEY]) {
        target[SINGLETON_KEY] = Reflect.construct(
          target,
          argumentsList,
          newTarget
        )
      }

      return target[SINGLETON_KEY]
    },
  })
}