課税所得に対する所得税額を返す (計算根拠: 国税庁タックスアンサーNo.2260 所得税の税率 https://www.nta.go.jp/taxes/shiraberu/taxanswer/shotoku/2260.htm)

.gitignore

node_modules/

incomeTax.js

/**
 * 課税所得に対する所得税額を返す (計算根拠: 国税庁タックスアンサーNo.2260 所得税の税率 https://www.nta.go.jp/taxes/shiraberu/taxanswer/shotoku/2260.htm)
 * @param {number} taxableIncome 課税所得
 * @returns {number} 所得税額
 */

function incomeTax(taxableIncome) {
    if (typeof taxableIncome != 'number') {
        throw TypeError('argument should be a number');
    }
    if (taxableIncome < 0 ) {
        throw TypeError('argument should be equal or above 0');
    }
    const CalculatingTable = [
        // [ threshold, taxRate(percent), deduction ]
        [ 40000000, 45, 4796000],
        [ 18000000, 40, 2796000],
        [  9000000, 33, 1536000],
        [  6950000, 23,  636000],
        [  3300000, 20,  427500],
        [  1950000, 10,   97500],
        [       -1,  5,       0]
    ];
    let matchedTuple = CalculatingTable.find(
        function(tuple) {
            let threshold = tuple[0];
            if (taxableIncome > threshold) {
                return true;
            }
        });
    if (matchedTuple === undefined) {
        throw 'Unknown error';
    }
    let taxRate = matchedTuple[1] / 100;
    let deduction = matchedTuple[2];
    return taxableIncome * taxRate - deduction;
}

module.exports = incomeTax;

package.json

{
  "name": "income-tax-jp",
  "version": "1.0.0",
  "description": "課税所得に対する所得税額を返す (計算根拠: 国税庁タックスアンサーNo.2260 所得税の税率 https://www.nta.go.jp/taxes/shiraberu/taxanswer/shotoku/2260.htm)",
  "main": "index.js",
  "scripts": {
    "test": "mocha"
  },
  "keywords": [],
  "author": "Hide Ishi (https://github.com/zerobase)",
  "license": "ISC",
  "devDependencies": {
    "mocha": "^6.1.4",
    "should": "^13.2.3"
  }
}

test.js

var incomeTax = require('./incomeTax.js');
require('should');

describe('incomeTax', function() {
    it('should return positive number', function() {
        let r = incomeTax(0);
        r.should.be.a.Number;
        r.should.aboveOrEqual(0);
    });
    it('should throw an error for non positive number', function() {
        (function(){incomeTax(-1)}).should.throw(/argument should be equal or above 0/);
    });
    it('should throw an error for non number', function() {
        (function(){incomeTax('1')}).should.throw(/argument should be a number/);
    });
    it('should return 974000 for 7000000', function() {
        incomeTax(7000000).should.be.eql(974000);
    });
});