Transform a function into the same function but with limited async calls at the same time.

limitAsyncCalls.js

const limitAsyncCalls = (fn, n) => {
  const cap = n;
  let executing_count = 0;
  let waitings = [];

  const wait = () => new Promise(res => waitings.push(res));
  const execute = async (fn, args) => {
    executing_count++;
    const result = await fn(...args);
    executing_count--;
    (waitings.shift() || (() => {}))();
    return result;
  };

  return async function(){
    if(executing_count >= cap) await wait();
    return await execute(fn, arguments);
  };
};

let i = 0;
const fn = limitAsyncCalls(async function(n){
  const id = i++;
  console.log(id + ' starts');
  return new Promise(res => setTimeout(() => {
    console.log(id + ' ends');
    res(id + n);
  }, Math.random() * 1000));
}, 2);

[0,1,2,3,4,5].map(fn);

/* Output:
0 starts
1 starts
0 ends
2 starts
1 ends
3 starts
2 ends
4 starts
3 ends
5 starts
4 ends
5 ends
*/