Calculate tax burden of IL joint filers in JS under the Fair Tax Amendment

ilTax.js

// gonna refer to this as bracket0, bracket1 etc

//| Income                 | Marginal Tax Rate |
//|------------------------|-------------------|
//| $0 – $10,000           | 4.75%             |
//| $10,001 – $100,000     | 4.90%             |
//| $100,001 – $250,000    | 4.95%             |
//| $250,001 – $500,000    | 7.75%             |
//| $500,001 – $1,000,000  | 7.85%             |
//| $1,000,001 and above   | 7.95% on net      |

const brackets = [
  { min: 0, max: 10000, rate: 0.0475 },
  { min: 10001, max: 100000, rate: 0.049 },
  { min: 100001, max: 250000, rate: 0.0495 },
  { min: 250001, max: 500000, rate: 0.0775 },
  { min: 500001, max: 1000000, rate: 0.0785 },
];

// assume income is Number
function calculateNetTaxForILJointFiler(income) {
  const truncatedIncome = Math.floor(income); // taxes are always calculated with income truncated

  let rtn = 0;
  if (truncatedIncome >= 1000001) rtn = truncatedIncome * 0.0795; // sighs non marginal

  // this is not performant in any way but that's OK
  for (const bracket of brackets) {
    const func = getBracketFunction(bracket.min, bracket.max, bracket.rate);
    const bracketRtn = func(truncatedIncome);
    rtn += bracketRtn.amount;
    if (bracketRtn.done) break;
  }

  return Math.floor(rtn);
}

const getBracketFunction = (min, max, rate) => {
  return (income) => {
    if (income < min) return { amount: 0, done: true };
    if (income > max) return { amount: rate * (max - min), done: false };

    // in between

    return { amount: rate * (income - min), done: true };
  };
};