function Semaphore(count = 1) {

  const resolvers = [];

  let startCount = count;

​

   return {

     aquire() {

       return new Promise(resolve => {

         if(startCount) { resolve(); startCount -= 1; }

         else resolvers.push(resolve);

       });

     },

     free() { 

       if(resolvers.length) resolvers.shift()(); 

       else startCount += 1;

     },

     async use(cb) { 

       await this.aquire(); 

       await cb(); 

       this.free() 

     },

     async done() {

       await Promise.all(Array.from({ length: count }, () => this.aquire()));

       startCount = count;

     },

   };

 }

​

const timer = ms => new Promise(resolve => setTimeout(resolve, ms));

​

(async function() {

  const s = Semaphore(3);

​

  for(let i = 0; i < 9; i++) {

    s.use(async () => {

     console.log(i + " start");

     await timer(5000 * Math.random());

     console.log(i + " end");

    });

  }

​

  await s.done();

  console.log("done");

})();