new_tax_bill.js

// https://www.reuters.com/article/us-usa-tax-provisions-factbox/whats-in-the-final-republican-tax-bill-idUSKBN1ED27K

// 10 percent up to $9,525, versus 10 percent up to $9,325 under existing law;
// 12 percent from $9,526 to $38,700, versus 15 percent on $9,326 to $37,950;
// 22 percent on $38,701 to $82,500, versus 25 percent on $37,951 to $91,900;
// 24 percent on $82,501 to $157,500, versus 28 percent on $91,901 to $191,650;
// 32 percent on $157,501 to $200,000, versus 33 percent on $191,651 to $416,700;
// 35 percent on $200,001 to $500,000, versus 35 percent on $416,701 to $418,400;

function calcTax(income) {
  const brackets = [
    { tax: 0.35, income: 200000 },
    { tax: 0.32, income: 157501 },
    { tax: 0.24, income: 82500 },
    { tax: 0.22, income: 38700 },
    { tax: 0.12, income: 9525 },
    { tax: 0.1, income: 0 },
  ];

  const oldBrackets = [
    { tax: 0.35, income: 416701 },
    { tax: 0.33, income: 191651 },
    { tax: 0.28, income: 91901 },
    { tax: 0.25, income: 37951 },
    { tax: 0.15, income: 9326 },
    { tax: 0.1, income: 0 },
  ];

  function getTax(brackets, income) {
    console.log('Calculating tax for brackets');

    let totalTax = 0;

    brackets.forEach(bracket => {
      const incomeInBracket = income - bracket.income;
      let taxInBracket;

      if (incomeInBracket > 0) {
        taxInBracket = incomeInBracket * bracket.tax;
        totalTax += taxInBracket;
      }

      console.log(
        `Income in bracket
          ${bracket.income} (of which you have ${Math.max(incomeInBracket, 0)})
          taxed at ${bracket.tax}
          is ${taxInBracket || 'none'}
        `
      );

      // clamp
      income = Math.min(income, bracket.income);
    });

    return totalTax;
  }

  const newTax = getTax(brackets, income);
  const oldTax = getTax(oldBrackets, income);

  return `
    On your income of ${income} you pay ${newTax.toFixed(2)} in taxes
    which is ~${(newTax / income).toFixed(2)}%.

    Before, you paid ${oldTax.toFixed(2)} in taxes
    which is ~${(oldTax / income).toFixed(2)}%`;
}

// usage:
// calcTax(65000)