This blog assumes a global install of mocha, although that can also be installed locally. chai is also required as the assertion library - install as a dev dependency.

Create fizzbuzz.mocha.js.

const lib = require("./index");
const chai = require("chai");
const { expect } = chai;
describe("check basic function", function () {
  it("should return when not part of fizz buzz", function () {
    const res = lib.run(2);
    expect(res).to.equal(2);
  });

  it("should return fizz", function () {
    const res = lib.run(3);
    expect(res).to.equal("Fizz");
  });

  it("should return buzz", function () {
    const res = lib.run(5);
    expect(res).to.equal("Buzz");
  });

  it("should return fizzbuzz", function () {
    const res = lib.run(15);
    expect(res).to.equal("FizzBuzz");
  });
});
copy

Create index.js:

const run = (arg) => {
  switch (true) {
    case arg % 15 === 0:
      return "FizzBuzz";
    case arg % 3 === 0:
      return "Fizz";
    case arg % 5 === 0:
      return "Buzz";
    default:
      return arg;
  }
};

module.exports = {
  run,
};
copy

Change into directory and run mocha fizzbuzz.mocha.js.