Simple NodeJS utility for calculating greek tax for 2020

tax.js

'use strict';

const readline = require('readline');

// Tax percentages per salary level
const taxLevels = [10000, 20000, 30000, 40000, Infinity];
const taxPercentages = [0.09, 0.22, 0.28, 0.36, 0.44];

// Eisfora percentages
const eisforaLevels = [12000, 20000, 30000, 40000, 65000, 220000, Infinity];
const eisforaPercentages = [0, 0.022, 0.05, 0.065, 0.075, 0.09, 0.1];

// Tax reduction per kids - salary combo
const reductionPerKids = [777, 810, 900, 1120, 1340];
const reductionPerExtraKid = 220;

const getReadLine = () =>  readline.createInterface({
    input: process.stdin,
    output: process.stdout
});

const getAnswer = (rl, prompt) => new Promise((resolve) => rl.question(prompt + ' ', resolve));

const getSalary = async (rl) => {
    const salaryText = await getAnswer(rl, 'Please input your taxable annual salary');
    return parseFloat(salaryText);
};

const getKids = async (rl) => {
    const kidsText = await getAnswer(rl, 'How many kids have you got?');
    return parseInt(kidsText, 10);
};

const getInsurance = async (rl) => {
    const insuranceText = await getAnswer(rl, 'How much insurance should be deducted?');
    return parseFloat(insuranceText);
};

const getReduction = (salary, kids = 0) => {
    const maxDefinedKidsInTable = reductionPerKids.length - 1;
    let reduction = kids > maxDefinedKidsInTable ? // more than specific kids from table
        reductionPerKids[maxDefinedKidsInTable] + (kids - maxDefinedKidsInTable) * 220 : // max reduction + 220 per extra kid
        reductionPerKids[kids]; // specific reduction
    if (salary > 12000 && kids < 5) { // no decrease in reduction for salaries under 12000 or more than 5 kids
        reduction -= ((salary - 12000) / 1000 * 20); // decrease reduction for 20 per 1000 extra in salary above 12000
    } 
    return Math.max(reduction, 0);
};

const calculatePercentageLevels = (amount, levelTables, percentageTables) => {
    let total = 0 ;
    for (let i = 0; i < levelTables.length; i++) { // browse tax levels
        const currentLevel = levelTables[i];
        const previousLevel = i === 0 ? 0 : levelTables[i-1]; // get previous taxLevel or 0 if start
        const currentPercentage = percentageTables[i];
        if (amount < currentLevel) { // in tax level range
            total += (amount - previousLevel) * currentPercentage;
            break; // currentLevel already bigger, no need to continue
        } else { // exceeds level
            total +=  (currentLevel - previousLevel) * currentPercentage;
        }
    }
    return total;
};

const calculateTax = (salary) => calculatePercentageLevels(salary, taxLevels, taxPercentages);

const calculateEisfora = (salary) => calculatePercentageLevels(salary, eisforaLevels, eisforaPercentages);

const getSalaryParts = (salary, insurance, kids) => {
    const taxableSalary = salary - insurance;
    const initialTax = calculateTax(taxableSalary);
    const eisfora = calculateEisfora(taxableSalary);
    const reduction = getReduction(taxableSalary, kids);
    const totalTax = Math.round((initialTax - reduction + eisfora) * 100) / 100; // round tax to two decimals
    const netSalary = salary - totalTax;
    return { taxableSalary, initialTax, eisfora, reduction, totalTax, netSalary };
};

const grossForNetSalary = (targetNetSalary, insurance, kids, threshold = 1, step = 1) => {
    let grossSalary = targetNetSalary;
    while (targetNetSalary - getSalaryParts(grossSalary, insurance, kids).netSalary > threshold) {
        grossSalary += step;
    }
    return grossSalary;
};

const main = async () => {
    const rl = getReadLine();
    const salary = await getSalary(rl);
    const insurance = await getInsurance(rl);
    const kids = await getKids(rl);

    const { taxableSalary, initialTax, eisfora, reduction, totalTax, netSalary } = getSalaryParts(salary, insurance, kids);
    // ------- Printing ----------
    console.log(`Tax calculation for ${salary} with ${kids} kids`);
    console.log('Taxable salary:', taxableSalary);
    console.log('Initial Tax:', initialTax);
    console.log('Eisfora:', eisfora);
    console.log('Tax Reduction:', reduction);
    console.log('Total tax:', totalTax);
    console.log('Net Salary:', netSalary);
    // Target Net salary
    const targetGrossSalary = grossForNetSalary(salary, insurance, kids);
    console.log('Gross salary needed to clear taxes:', targetGrossSalary);
    console.log('Virtual tax difference:', targetGrossSalary - salary);
    rl.close();
};

main()
    .then(() => {
        process.exit(0);
    })
    .catch(err => {
        console.error('Error with operation', err);
        process.exit(1);
    });