Limiting the number of concurrent running promises

max-concurrent-promises.js

let runningPromiseCount = 0;

const examplePromiseRunnerQueue = new Array(10)
  .fill(0)
  .map((_, idx) => idx)
  .map((idx) => {
    return async () => {
      console.log(
        `[${new Date().toISOString()}] Promise ${idx} will run! \n# of running promises: ${runningPromiseCount}`
      );

      runningPromiseCount++;

      return await new Promise((resolve) => {
        setTimeout(() => {
          resolve(idx);
          runningPromiseCount--;
        }, idx * 1000);
      });
    };
  });

const run = (promiseRunnerQueue = [], max = promiseRunnerQueue.length) => {
  const executeAndDelegateNextCall = async (promiseRunner) => {
    try {
      await promiseRunner();
    } finally {
      if (promiseRunnerQueue.length === 0) {
        return;
      }

      const nextPromiseRunner = promiseRunnerQueue.shift();
      executeAndDelegateNextCall(nextPromiseRunner);
    }
  };

  promiseRunnerQueue.splice(0, max).forEach(executeAndDelegateNextCall);
};

run(examplePromiseRunnerQueue, 10);

Example trace output:

[2021-04-29T21:26:31.893Z] Promise 0 will run! 
# of running promises: 0 
[2021-04-29T21:26:31.894Z] Promise 1 will run! 
# of running promises: 1 
[2021-04-29T21:26:31.894Z] Promise 2 will run! 
# of running promises: 2 
[2021-04-29T21:26:31.894Z] Promise 3 will run! 
# of running promises: 3 
[2021-04-29T21:26:31.894Z] Promise 4 will run! 
# of running promises: 4 
[2021-04-29T21:26:31.904Z] Promise 5 will run! 
# of running promises: 4 
[2021-04-29T21:26:32.894Z] Promise 6 will run! 
# of running promises: 4 
[2021-04-29T21:26:34.446Z] Promise 7 will run! 
# of running promises: 4 
[2021-04-29T21:26:35.446Z] Promise 8 will run! 
# of running promises: 4 
[2021-04-29T21:26:36.446Z] Promise 9 will run! 
# of running promises: 4