parallelLimit.js

const tasks = [
  1,1,1,1,,1,1,1,1
  ,1,1,1,2,2,2,2,3,
  3,,3,43,4343,43,
  ,43,4,3,4,34,3,4,3
]
.filter(Boolean)
.map(t => cb => {
  setTimeout(cb, 1000, 1,2);
});

const parallelLimit = (tasks, limit, done) => {
  let running = 0;
  let results = [];
  (function run(){
    while(running < limit && tasks.length){
      running++;
      const task = tasks.shift();
      task((err, res) => {
        running--;
        results.push({ res, err });
        if(tasks.length){
          run();
        }else{
          done(results);
        }
      });
    }
  })();
};

parallelLimit(tasks, 5, r => {
  console.log(r);
});