Final calculations for ROI calculator

//calculates the new bill based on a number and a percent savings that you feed it.
function newBill(bill, percentSavings){
   var result = -1*(bill*percentSavings)+bill;
   return result;  
}


function finalCalculationWithCheckedProducts(array, originalBill){
  var stopCase = array.length -1;
  var i=0;
  var averageMonthlyBill = originalBill;
  for (i=0;i<=stopCase;i++)
  {
   var iterateBill = newBill(averageMonthlyBill,array[i]);
   averageMonthlyBill = iterateBill;
  }
  //ends the for loop
  return averageMonthlyBill;
}


//tests for this code



//final calculation tests
describe("calculation for savings", function(){
  it("in the case that we have a $300 bill with a 20% savings the new bill (after savings) should be 240", function(){
    var bill = 300;
    var percentSavings = 0.2;
    expect(newBill(bill, percentSavings)).toEqual(240);
  });
});

//final calculation tests
describe("Final calculation", function(){
  it("with a average monthly bill of $300 and purchase of products that save 20%, 5%, and 10% anually; the new monthly average should be 205.2", function(){
    var percentages = [0.2,0.05,0.10];
    var bill = 300;
    expect(finalCalculationWithCheckedProducts(percentages,bill)).toEqual(205.2);
  });
});

//final calculation tests
describe("The order of percentages in array should not matter.", function(){
  it("with a average monthly bill of $300 and purchase of products that save 5%, 20%,and 10% anually; the new monthly average should be 205.2", function(){
    var percentages = [0.05,0.2,0.10];
    var bill = 300;
    expect(finalCalculationWithCheckedProducts(percentages,bill)).toEqual(205.2);
  });
});