Calculate tax from brackets

calculate_tax.js

const MAX = Number.MAX_SAFE_INTEGER
const BRACKETS = {
  AU_2025: [
    [ 0,        18200,   0.00 ],
    [ 18201,    45000,   0.16 ],
    [ 45001,    135000,  0.30 ],
    [ 135001,   190000,  0.37 ],
    [ 190001,   MAX,     0.45 ],
  ],
  AU_2024: [
    [ 0,        18200,   0.00 ],
    [ 18201,    45000,   0.19 ],
    [ 45001,    120000,  0.325],
    [ 120001,   180000,  0.37 ],
    [ 180001,   MAX,     0.45 ],
  ]
};

function calculate_tax(
  income,
  brackets = BRACKETS.AU_2025,
) {
  let current = 0
  let remainingIncome = income

  for (const [ bracketStart, bracketEnd, taxRatePercent, additionalCharge = 0 ] of brackets) {
    const bracketSize = bracketEnd - bracketStart
    const overflow = remainingIncome - bracketSize

    if (overflow >= 0) {
      current += bracketSize * taxRatePercent
      current += additionalCharge
      remainingIncome = overflow
    } else {
      current += remainingIncome * taxRatePercent
      current += additionalCharge
      break
    }

    remainingIncome = overflow
  }

  return parseInt(current.toFixed(0), 10)
}