Tiny Javascript Semaphore

semaphore.js

function Semaphore(count) {
  this.permits = count;
  this.queue = [];
}

Semaphore.prototype.acquire = function() {
  return new Promise(resolve => {
    if (0 === this.permits)
      this.queue.push(resolve);
    else {
      this.permits--;
      resolve();
    }
  });
};

Semaphore.prototype.release = function() {
  const resolve = this.queue.shift();
  if (resolve)
    resolve();
  else
    this.permits++;
};