Adds functions to calculate income tax and national insurance on raw income

HMRC_IncomeTax_NIC_ExcelOnline.ts

/** @CustomFunction */
function incomeTax(pay: number): number {

  var additionalTax = 0;
  var higherTax = 0;
  var basicTax = 0;

  var personalAllowanceToRemove = Math.floor((pay - 100000) / 2);
  var defaultPersonalAllowance = 12570;

  var personalAllowance = defaultPersonalAllowance;
  if (personalAllowanceToRemove > 0) {
    personalAllowance -= personalAllowanceToRemove
    personalAllowance = Math.max(personalAllowance, 0)
  }

  if (pay < personalAllowance) {
    return 0;
  }

  var taxableAmount = pay - personalAllowance;

  var additionalAmountThreshold = (125140)
  var additionalAmount = taxableAmount - additionalAmountThreshold;

  if (taxableAmount > additionalAmountThreshold) {
    additionalTax = 0.45 * additionalAmount;
    taxableAmount = additionalAmountThreshold;
  }

  var higherAmountThreshold = (50270 - defaultPersonalAllowance)
  var higherAmount = taxableAmount - higherAmountThreshold;
  if (taxableAmount > higherAmountThreshold) {
    higherTax = 0.4 * higherAmount;
    taxableAmount = higherAmountThreshold
  }

  var basicAmount = taxableAmount;
  if (basicAmount > 0) {
    basicTax = 0.2 * basicAmount;
  }
  console.log("persallowance:" + personalAllowance + "additional: " + additionalTax + " higher:" + higherTax + " basic: " + basicTax)
  var totalTax = basicTax + higherTax + additionalTax
  return totalTax
}

/** @CustomFunction */
function nationalInsuranceClass1(pay: number): number {
  var twelvePercentThreshold = 1048 * 12;
  var twoPercentThreshold = 4189 * 12;
  var nic = 0;

  var applicableAmount = pay;

  if (applicableAmount > twoPercentThreshold) {
    nic += (applicableAmount - twoPercentThreshold) * 0.02
    applicableAmount = twoPercentThreshold;
  }
  if (applicableAmount > twelvePercentThreshold) {
    nic += (applicableAmount - twelvePercentThreshold) * 0.12;
  }

  return nic;
}