Parallel Job Runner

1-parallel-job-runner.js

/**
 * Creates a function that performs any promisable task in parallel.
 * @param {number} [concurrency] - The maximum number of concurrent jobs that are able to run simultaneously.
 * @return {Promise}
 */
const createJobRunner = (concurrency = 3) => {

    const EMPTY_SLOT = false;
    const BUSY_SLOT = true;

    const workersQueue = [];
    const workingSlots = new Array(concurrency).fill(EMPTY_SLOT);

    const next = (availableSlot = workingSlots.indexOf(EMPTY_SLOT)) => {
        if (workersQueue.length === 0 || availableSlot < 0) return;

        workingSlots[availableSlot] = BUSY_SLOT;
        const worker = workersQueue.shift();
        worker(() => {
            workingSlots[availableSlot] = EMPTY_SLOT;    
            next(availableSlot)
        });
    }

    return (worker) => new Promise( (resolve, reject) => {
        workersQueue.push((onFinish) => {
            const job = worker();
            if (job instanceof Promise) {
                job.then(resolve).catch(reject).finally(onFinish);
            } else {
                try {
                    resolve(job);
                } catch (e) {
                    reject(e);
                } finally {
                    onFinish();
                }
            }
        });
        next();
    });
}

2-example.js

const doWork = createJobRunner(5);

for (let i=0; i<10; i++) {
  doWork(() => new Promise((resolve) => {
    console.log(`Job ${i} started`); 
    setTimeout(() => {
      console.log(`Job ${i} done`);
      resolve();
    }, Math.random() * 1000)
  }));
}

//// Possible output:
// Job 0 started
// Job 1 started
// Job 2 started
// Job 3 started
// Job 4 started
// Job 1 done
// Job 5 started
// Job 5 done
// Job 6 started
// Job 2 done
// Job 7 started
// Job 7 done
// Job 8 started
// Job 4 done
// Job 9 started
// Job 3 done
// Job 0 done
// Job 8 done
// Job 6 done
// Job 9 done