tax-rate.js

const beijing = {
  social_fund: {
    min: 2820,
    max: 14096,
    // rate: .105
    rate: {
      medical: .02,
      endowment: .08,
      unemployment: .005
    }
  },
  accumulation_fund: {
    min: 2010,
    max: 24311,
    rate: .12
  },
  table: [
    { level: 1, min: 0,      max: 36000,    rate: 0.03, quickCalculateDeducting: 0      },
    { level: 2, min: 36000,  max: 144000,   rate: 0.10, quickCalculateDeducting: 2520   },
    { level: 3, min: 144000, max: 300000,   rate: 0.20, quickCalculateDeducting: 16920  },
    { level: 4, min: 300000, max: 420000,   rate: 0.25, quickCalculateDeducting: 31920  },
    { level: 5, min: 420000, max: 660000,   rate: 0.30, quickCalculateDeducting: 52920  },
    { level: 6, min: 660000, max: 960000,   rate: 0.35, quickCalculateDeducting: 85920  },
    { level: 7, min: 960000, max: Infinity, rate: 0.45, quickCalculateDeducting: 181920 },
  ]
};

const range = (v, { min, max }) => {
  return Math.min(Math.max(v, min), max);
};

const inRange = (v, rule) => {
  return (v > rule.min && v <= rule.max) && rule;
};

const calcSocialfund = (money, profile = beijing.social_fund) => {
  const { rate } = profile;
  const base = range(money, profile);
  const medical = base * rate.medical;
  const endowment = base * rate.endowment;
  const unemployment = base * rate.unemployment;
  return {
    base,
    money,
    medical,
    endowment,
    unemployment,
    patTax: medical + endowment + unemployment
  };
};

const calcAccumulationFund = (money, profile = beijing.accumulation_fund) => {
  const { rate } = profile;
  const base = range(money, profile);
  return {
    base,
    money,
    patTax: base * rate
  };
};

const taxRate = ({
  salary,
  deducted = 5000,
  specialDeducted,
  sixSpecialDeducted
}, profile) => {
  const { table } = profile;
  specialDeducted = specialDeducted || [
    calcSocialfund(salary),
    calcAccumulationFund(salary)
  ].reduce((a,b) => a+b.patTax, 0);
  const byMonth = month => {
    const x
    = (salary * month)
    - (deducted * month)
    - (specialDeducted * month) 
    - (sixSpecialDeducted * month);
    const rule = table.find(rule => inRange(x, rule));
    var historyAmount = 0;
    if(month > 1){
      historyAmount = [...new Array(month - 1)]
        .reduce((amount, _, i) => amount + byMonth(i+1), 0);
    }
    return x * rule.rate - rule.quickCalculateDeducting - historyAmount;
  };
  return byMonth;
};

const calc = taxRate({
  salary: 38060,
  // deducted: 5000,
  // specialDeducted: 4500,
  sixSpecialDeducted: 2000,
}, beijing);

const months = [...Array(12).keys()].map(m => m + 1);
months.forEach(month => {
  console.log('=>', month, calc(month));
});

➜ node ~/Desktop/tax-rate.js
=> 1 555
=> 2 625
=> 3 1850
=> 4 1850
=> 5 1850
=> 6 1850
=> 7 1850
=> 8 2250
=> 9 3700
=> 10 3700
=> 11 3700
=> 12 3700