Parallel.js

//implement missing "Parallel" please
//try to do so with a minimal amount of code

function Parallel (options) {
  var parallelJob = options.parallelJob || 1;
  var jobs = [];
  var result = [];

  function runJobs() {
    return new Promise(function(resolve) {
      var jobsToRun = jobs.splice(0, parallelJob);
      var runningJobs = [];
      for (var key in jobsToRun) {
        runningJobs.push(new Promise(function (resolve) {
          jobsToRun[key](resolve);
        }));
      }
      Promise.all(runningJobs).then(function (values) {
        values.forEach(function(item){
          result.push(item);
        });
        if (jobs.length > 0) {
          runJobs().then(function () {
            resolve(result);
          })
        }
        else {
          resolve(result);
        }
      })
    });
  }

  return {
    job: function (step) {
      jobs.push(step);
      return this;
    },
    done: function (callback) {
      runJobs().then(function(values) {
        callback(values);
      });
    }
  }
}

var runner = new Parallel({ parallelJob: 2 });

runner
  .job(step1)
  .job(step2)
  .job(step3)
  .job(step4)
  .done(onDone);

function step1(done) {
  console.log('step1');

  setTimeout(done, 10000, 'step1');
}

function step2(done) {
  console.log('step2');

  setTimeout(done, 1000, 'step2');
}

function step3(done) {
  console.log('step3');

  setTimeout(done, 5000, 'step3');
}

function step4(done) {
  console.log('step4');

  setTimeout(done, 7000, 'step4');
}

function onDone(results) {
  console.log(results);
  console.assert(Array.isArray(results), 'expect result to be array');
  console.assert(results[0] === 'step1', 'Wrong answer 1');
  console.assert(results[1] === 'step2', 'Wrong answer 2');
  console.assert(results[2] === 'step3', 'Wrong answer 3');
  console.assert(results[3] === 'step4', 'Wrong answer 4');
}