Limiting the number of concurrent running promises streamed via a generator

max-concurrent-promises-gen.js

const DEFAULT_MAX_CONCURRENT_PROMISES = 5;

/**
 * Runs a stream of promises and ensures only `max` promises are run at a time
 * @param {*} promiseRunnerGen
 * @param {*} max
 */
const run = (promiseRunnerGen, max = DEFAULT_MAX_CONCURRENT_PROMISES) => {
  const iterator = promiseRunnerGen();
  let runningPromiseCount = 0;

  const executeAndDelegateNextCall = async (promiseRunner) => {
    try {
      runningPromiseCount++;
      
      await promiseRunner();
    } finally {
      runningPromiseCount--;

      const { value: nextPromiseRunner, done } = iterator.next();

      if (!done) {
        executeAndDelegateNextCall(nextPromiseRunner);
      }
    }
  };

  let current = iterator.next();
  
  while (!current.done && runningPromiseCount < max) {
    executeAndDelegateNextCall(current.value);
    current = iterator.next();
  }
};

export { run };