testRunner.js

const describe = (description, callback) => {
  console.log(description);
  callback();
};

const it = (message, callback) => describe(` ${message}`, callback);

const matchers = expression => ({
  toBe: assertion => {
    if (expression === assertion) {
      console.log("PASS");
      return true;
    } else {
      console.log("FAIL");
      return false;
    }
  }
});

const expect = expression => matchers(expression);

function multiplier(a, b) {
  return a * b;
}

describe("multiplier", () => {
  it("multiplies two numbers", () => {
    const result = multiplier(5, 2);
    expect(result).toBe(10);
  });
});