CountDownLatch in typescript

CountDownLatch.spec.ts

import {describe, it} from 'mocha';
import {equal, throws } from 'assert';
import { CountDownLatch } from './CountDownLatch';

describe('CountDownLatch usage suite', () => {

  it('fails correctly with a negative starting count', () => {
    throws(() => {
      new CountDownLatch(-1);
    }, 'should have thrown an error with a negative start count');
  });

  it('counts down from 1 correctly', (done: Mocha.Done) => {
    const latch = new CountDownLatch(1);
    equal(latch.count, 1);
    latch.onZero().then(() => {
      equal(latch.count, 0);
      done();
    });
    setTimeout(() => {
      latch.countDown();
    }, 100);
    equal(latch.count, 1);
  });

  it('counts down from 3 correctly', (done: Mocha.Done) => {
    const latch = new CountDownLatch(3);
    equal(latch.count, 3);
    latch.onZero().then(() => {
      equal(latch.count, 0);
      done();
    });
    setTimeout(() => {
      latch.countDown();
      equal(latch.count, 2);
    }, 100);
    setTimeout(() => {
      latch.countDown();
      equal(latch.count, 1);
    }, 200);
    setTimeout(() => {
      latch.countDown();
      equal(latch.count, 0);
    }, 300);
    setTimeout(() => {
      latch.countDown();
      // does not go below zero
      equal(latch.count, 0);
    }, 400);
    equal(latch.count, 3);
  });

});

CountDownLatch.ts

export class CountDownLatch {

  private _count: number;
  private _onZeroPromise: Promise<void>;
  private _reachedZero: ((value?: void | PromiseLike<void> | undefined) => void) | undefined;

  constructor(count: number) {
    if ( count < 0 ) {
      throw new Error('count cannot be negative');
    }
    this._count = count;
    this._onZeroPromise = new Promise((resolve) => {
      this._reachedZero = resolve;
    });
  }

  public countDown() {
    if ( this._count > 0 ) {
      this._count -= 1;
      if ( this._count == 0 && undefined != this._reachedZero ) {
        this._reachedZero();
      }
    }
  }

  public onZero(): Promise<void> {
    return this._onZeroPromise;
  }

  public get count() {
    return this._count;
  }

}