Tax.js

/* 
 * Copyright (c) 2020-2021 Trevor Caira
 *
 * Permission is hereby granted, free of charge, to any person obtaining a
 * copy of this software and associated documentation files (the "Software"),
 * to deal in the Software without restriction, including without limitation
 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
 * and/or sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

const FEDERAL_BRACKETS = [
  [0, .1],
  [9700, .12],
  [39475, .22],
  [84200, .24],
  [160725, .32],
  [204100, .35],
  [510300, .37]
];
const STANDARD_DEDUCTION = 12500;
const FICA_MAX_INCOME = 142800;
const FICA_RATE = 0.062;
const MEDICARE_RATE = 0.0145;
const MEDICARE_SURTAX_EXEMPTION = 200000;
const MEDICARE_SURTAX_RATE = 0.009;
const SALT_CAP = 10000;

const CO_RATE = 0.0463;
const NC_RATE = 0.0525;
const PA_RATE = 0.0307;

const CA_BRACKETS = [
  [0, 0.01],
  [8809, 0.02],
  [20883, 0.04],
  [32960, 0.06],
  [45753, 0.08],
  [57824, 0.093],
  [295373, 0.103],
  [354445, 0.113],
  [590742, 0.123],
  [1000000, 0.133]
];

const CT_BRACKETS = [
  [0, 0.03],
  [10000, 0.05],
  [50000, 0.055],
  [100000, 0.06],
  [200000, 0.065],
  [250000, 0.069],
  [500000, 0.0699]
];

const NJ_BRACKETS = [
  [0, 0.014],
  [20000, 0.0175],
  [35000, 0.035],
  [40000, 0.0553],
  [75000, 0.0637],
  [500000, 0.0897],
  [5000000, 0.1075]
];

const NYS_BRACKETS = [
  [0, .04],
  [8500, 0.045],
  [11700, .0525],
  [13900, 0.059],
  [21400, 0.0621],
  [80650, 0.0649],
  [215400, 0.0685],
  [1077550, 0.0882]
];
const NYC_BRACKETS = [
  [0, 0.03078],
  [12000, 0.03762],
  [25000, 0.03819],
  [50000, 0.03876]
];

function progressiveIncomeTax(brackets, income) {
  let tax = 0;

  for (let i = 0; i < brackets.length - 1; ++i) {
    const bracketWidth = brackets[i+1][0] - brackets[i][0];
    const bracketIncome = Math.min(
      Math.max(income - brackets[i][0], 0),
      bracketWidth
    );
    tax += brackets[i][1] * bracketIncome;
  }

  const [lastThreshold, lastRate] = brackets[brackets.length - 1];
  tax += Math.max(0, income - lastThreshold) * lastRate;

  return tax;
}

function federalIncomeTax(income) {
  return progressiveIncomeTax(FEDERAL_BRACKETS, income);
}

function fica(grossIncome) {
  return Math.min(grossIncome, FICA_MAX_INCOME) * FICA_RATE;
}

function medicare(grossIncome) {
  return grossIncome * MEDICARE_RATE + Math.max(0, grossIncome - MEDICARE_SURTAX_EXEMPTION) * MEDICARE_SURTAX_RATE;
}

function federalTax(grossIncome, deductions) {
  return federalIncomeTax(grossIncome - Math.max(deductions, STANDARD_DEDUCTION)) + fica(grossIncome) + medicare(grossIncome);
}

function flatTax(rate, exemption, income, deductions) {
  return rate * Math.max(0, income - exemption - deductions);
}

function stateAndFederalTax(grossIncome, deductions, stateTax) {
  const itemizedDeductions = Math.min(stateTax, SALT_CAP) + deductions;
  return stateTax + federalTax(grossIncome, itemizedDeductions);
}

function coTax(income, deductions) {
  const stateTax = flatTax(CO_RATE, 0, income, deductions);
  return stateAndFederalTax(income, deductions, stateTax);
}

function paTax(income, deductions) {
  const stateTax = flatTax(0.01 + PA_RATE, 0, income, deductions);
  return stateAndFederalTax(income, deductions, stateTax);
}

function paTaxNoLocal(income, deductions) {
  const stateTax = flatTax(PA_RATE, 0, income, deductions);
  return stateAndFederalTax(income, deductions, stateTax);
}

function caTax(income, deductions) {
  const stateTax = progressiveIncomeTax(CA_BRACKETS, income - deductions);
  return stateAndFederalTax(income, deductions, stateTax);
}

function ctTax(income, deductions) {
  const stateTax = progressiveIncomeTax(CT_BRACKETS, income - deductions);
  return stateAndFederalTax(income, deductions, stateTax);
}

function njTax(income, deductions) {
  const stateTax = progressiveIncomeTax(NJ_BRACKETS, income - deductions);
  return stateAndFederalTax(income, deductions, stateTax);
}

function ncTax(income, deductions) {
  const stateTax = flatTax(NC_RATE, 0, income, deductions);
  return stateAndFederalTax(income, deductions, stateTax);
}

function nycTax(income, deductions) {
  const localTax = progressiveIncomeTax(NYC_BRACKETS, income);
  const stateTax = progressiveIncomeTax(NYS_BRACKETS, income);
  return stateAndFederalTax(income, deductions, localTax + stateTax);
}