Async Tasks running

tasks.js

function Runner() {
 // Write your code here
 // Amit's code start
 function runFunctions() {
    // Getting arguments from prototype
    var tasks = Array.prototype.concat.apply([], arguments);
    //console.log(tasks, "functions lists");
    // Shifing tasks functions into a que
    var task = tasks.shift();
    // Executing one by one
    task(function() {
        if(tasks.length > 0)
        // if there are functions exist in tasks list make call and callback one by one
            //console.log(tasks, "Final in");
            runFunctions.apply(this, tasks);
    });
}

 // Final call of functions
 runFunctions(f1, f2, f3, f4);

 // Amit's code end
 
}

function f1(done) {
  setTimeout(function() {
    console.log('Executing f1');
    done();
  }, 100);
}


function f2(done) {
  setTimeout(function() {
    console.log('Executing f2');
    done();
  }, 200);
}


function f3(done) {
  setTimeout(function() {
    console.log('Executing f3');
    done();
  }, 20);
}


function f4(done) {
  setTimeout(function() {
    console.log('Executing f4');
    done();
  }, 10);
}

var runner = new Runner();

runner.run(f1);
runner.run(f2);
runner.run(f3);
runner.run(f4);