String Calculator - https://katayuno-app.herokuapp.com/katas/5

calculator.js

const CUSTOM_DELIMITER_PATTERN = "//";
const DEFAULT_DELIMITER = ",";
const DELIMITIER_POSITION = 2;
const NUMBERS_START_POSITION = 4;

const parseInput = (input) => {
  if (input.startsWith(CUSTOM_DELIMITER_PATTERN)) {
    return {
      delimiter: input[DELIMITIER_POSITION],
      rawNumbers: input.substring(NUMBERS_START_POSITION, input.length)
    };
  }
  return {
    delimiter: DEFAULT_DELIMITER,
    rawNumbers: input
  };
};

const calculator = (input) => {
  if (input.length === 0) return 0;
  const { delimiter, rawNumbers } = parseInput(input);
  if (rawNumbers.includes("-")) {
    throw Error("negatives not allowed");
  }

  const numbers = rawNumbers.replaceAll("\n", ",").split(delimiter);

  return numbers.reduce((prev, next) => parseInt(prev) + parseInt(next), 0);
};

describe("Calculator", () => {
  it("simple case", () => {
    const result = calculator("");

    expect(result).toBe(0);
  });

  it("one number", () => {
    const result = calculator("1");

    expect(result).toBe(1);
  });

  it("couple of numbers", () => {
    const result = calculator("1,3");

    expect(result).toBe(4);
  });

  it("unknown amount of numbers", () => {
    const result = calculator("1,3,5,5,20");

    expect(result).toBe(34);
  });

  it("handle new lines between numbers", () => {
    const result = calculator("1\n2,3");

    expect(result).toBe(6);
  });

  it("support different delimiters", () => {
    const result = calculator("//;\n1;2");

    expect(result).toBe(3);
  });

  it("negative numbers", () => {
    expect(() => calculator("1,4,-1")).toThrow("negatives not allowed");
  });
});