Limit execution concurrency of promise generation functions

concurrent_promise.js

let allConcurrent = function(limit, generators){
  return new Promise(function(resolveAll){
    let plan = generators.map((generator)=>({ value: undefined, executed: false, settled: false, generator: generator }));
    let executeBatch = () => {
      plan
        .filter(({ settled, executed })=> !executed || (executed && !settled))
        .slice(0, limit)
        .filter(({ executed }) => !executed)
        .map(planItem => {
          planItem.executed = true;
          var next = (val) => { Object.assign(planItem, { settled: true, value: val}); executeBatch(); }
          planItem.generator().then(next, next);
        })
        .length || (plan.every(({ settled }) => settled) && resolveAll(plan.map(({value}) => value)));
    } 
    executeBatch();
  });
}