Resolve promises serially, with a given number in parallel

serial-resolve.js

// @flow

export const serialResolve = (functionsReturningPromises: Array<() => Promise<any>>, parallelTasks: number) => new Promise((resolve, reject) => {
  let tasksStarted = 0;
  // Create empty results array
  const results = Array.from(functionsReturningPromises, () => null);
  const totalFunctions = functionsReturningPromises.length;

  // When a promise completes, set the correct results index and start any additional tasks
  let completedPromises = 0;
  const promiseCompleted = (index:number) => (result: any) => {
    results[index] = result;
    completedPromises += 1;
    if (completedPromises === totalFunctions) {
      resolve(results);
    }
    if (tasksStarted < totalFunctions) {
      tasksStarted += 1;
      functionsReturningPromises[tasksStarted - 1]()
        .then(promiseCompleted(tasksStarted - 1))
        .catch(reject);
    }
  };

  // Run the first PARALLEL_TASKS tasks
  for (let i = 0;i < parallelTasks; i += 1) {
    tasksStarted += 1;
    functionsReturningPromises[i]()
      .then(promiseCompleted(tasksStarted - 1))
      .catch(reject);
  }
});

export default serialResolve;

if (!module.parent) {
  const compute = (a) => new Promise(resolve => {
    setTimeout(() => {
      console.log(`COMPUTED TASK ${a}`);
      resolve(a + 1);
    }, Math.random() * 500);
  });

  // Use like this
  // const tasks = [0,1,2,3,4,5,6,7].map(a => () => compute(a))
  // serialResolve(tasks, 3).then(console.log);

  // Or like this
  const taskParams = [0, 1, 2, 3, 4, 5, 6, 7];
  serialResolve(taskParams.map((a) => () => compute(a)), 3).then(console.log);
}