simple Semaphore implementation in JavaScript

semaphore.js

class Semaphore {
    constructor(max) {
        this.counter = 0;
        this.waiting = [];
        
        this.max = max;
    }

    acquire() {
        if (this.counter < this.max) {
            this.counter++;
            
            return new Promise((resolve) => resolve());

        } else {
            return new Promise((resolve, err) => {
                this.waiting.push({
                    resolve: () => {
                        this.counter++;
                        resolve();
                    },
                    err: err
                });
            });
        }
    }

    isAvailable() {
        return this.counter < this.max;
    }

    release() {
        this.counter--;

        if (this.waiting.length > 0 && this.counter < this.max) {
            let promise = this.waiting.shift();
            promise.resolve();
        }
    }

    purge() {
        let unresolved = this.waiting.length;

        for (let i = 0; i < unresolved; i++) {
            this.waiting[i].err('Task has been purged.');
        }

        this.counter = 0;
        this.waiting = [];

        return unresolved;
    }
}

test.js

let sema = new Semaphore(1);

async function test(id) {
  console.log('queueing task', id);
  try {
    await sema.acquire();
    console.log('running task', id);

    setTimeout(() => {
        console.log('release task', id);
        sema.release();
    }, 2000);

  } catch (e) {
    console.error(id, e);
  }
}

test(1);
test(2);
test(3);
test(4);
test(5);

setTimeout(() => {
  test(10);
  test(11);
  test(12);
}, 1500);

setTimeout(() => {
  test(20);
  test(21);
  test(22);
}, 2700);