Promise pool to limit concurrency

promisePool.js

const promisePool = async function(functions, n) {
async function evaluateNext(){
  if(functions.length == 0 ) return ;
  const fn = functions.shift();
  await fn();
  await evaluateNext();   // call the function recursively until the function ends
}
const nPromises = Array(n).fill().map(evaluateNext);   // create array of size n to call the evalute next fn.
await Promise.all(nPromises);  // wait for all promise to resolve
};