Example of a function which takes a string of numbers separated by a specific delimiter and sums the numbers. Includes tests

string-calculator.test.ts

import {sumNumbers} from "../core/string-calculator";

describe('StringCalculator', () => {
    it('can handle null and empty strings', () => {
       expect(sumNumbers(null)).toBe(0);
       expect(sumNumbers('')).toBe(0);
    });

    it('can handle one number', () => {
        expect(sumNumbers('18')).toBe(18);
    });

    it('can handle numbers separated by commas', () => {
        expect(sumNumbers('5,10,15')).toBe(30);
    });

    it('can handle non-numeric values', () => {
       expect(sumNumbers('a')).toBe(0);
       expect(sumNumbers('8,a,10')).toBe(18);
    });

    it('can handle custom separators', () => {
       expect(sumNumbers('//%/5%2')).toBe(7);
       expect(sumNumbers('//%/4,6')).toBe(0);
       expect(sumNumbers('//#/4#6')).toBe(10);
    });
});

string-calculator.ts

const DEFAULT_SEPARATOR = ',';
const CUSTOM_SEPARATOR_REGEX = /^\/\/(.)\//;

export function sumNumbers(input: string) {
    if (!input) return 0;

    const separator = extractSeparator(input);
    const numbersString = getNumbersString(input);

    return numbersString.split(separator).reduce(sum, 0);
}

function sum(accumulator: number, currentToken: string) {
    return accumulator + parseTokenToNumber(currentToken);
}

function parseTokenToNumber(token: string) {
    const parsedToken = Number(token);
    return isNaN(parsedToken) ? 0 : parsedToken;
}

function extractSeparator(input: string) {
    const customSeparatorMatch = input.match(CUSTOM_SEPARATOR_REGEX);
    return customSeparatorMatch ? customSeparatorMatch[1] : DEFAULT_SEPARATOR;
}

function getNumbersString(input: string) {
    return input.replace(CUSTOM_SEPARATOR_REGEX, '');
}