my first async lib

app.js

magic([taskA, taskB, taskC], [taskD], [taskE, taskF], [taskG]); // see magic.js main()
/*
  each task is a function like this:
  
  function taskA(param1, param2, param3 ) { // arbitary number of params
    return function (fCallback) {
      setTimeout(function(){
        fCallback (null, param1);  // error, result
      }, param1)
    };
  }

  The tasks execute in this scenario
  
  1. a,b,c simultaneously
  2. d
  3. e,f simultaneously
  4. g -- you can use last task as a callback from this whole operation

*/

magic.js

"use strict";

function serial(arr) {
    //console.log("serial started");
    function next(err, result) {
        //console.log('serialResult\n\t', result);
        //console.log(" --> next");
        if (err) {
            throw err;
        }
        var currentTask = arr.shift();
        if (currentTask) {
            currentTask(next);
        }
    }
    next(null,  null);
}
function getParallelDispatcher(nTasks, fCallback) {
    var results = [];
    var errors = [];
    var nT = nTasks;
    var nC = 0;
    return function (err, result) {
        results.push(result);
        errors.push(err);
        nC++;
        if (nC === nT) {
            fCallback(errors, results);
        }
    };
}
function parallel(arr, fCallback) {
    var i,  iErrCntr, allNull = true;
    function onComplete(errs, results) {
        //console.log("parallel ended");
        for (iErrCntr = 0; iErrCntr < errs.length; iErrCntr++) {
            if (errs[iErrCntr]) {
                allNull = false;
            }
        }
        if (allNull) {
            fCallback(null, results);
        } else {
            fCallback(errs, results);
        }
    }
    //console.log("parallel started");
    var dispatcher = getParallelDispatcher(arr.length, onComplete);
    for  (i = 0; i < arr.length; i++) {
        arr[i](dispatcher);
    }
}


function getNewParallel(tasks) {
    var tsk = tasks;
    return function (next) {
        parallel(tsk, next);
    };
}
function main() {
    var tasks = [], iArgCntr;
    for (iArgCntr = 0; iArgCntr < arguments.length; iArgCntr++) {
        if (arguments[iArgCntr] instanceof Array) {
            tasks.push(getNewParallel(arguments[iArgCntr]));
        } else {
            tasks.push(getNewParallel([arguments[iArgCntr]]));
        }
    }
    serial(tasks);
}


module.exports = main;