Challenge 1: Test a Pure Function — Possible Solution
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// currency.ts
function formatCurrency(amount: number): string {
  const sign = amount < 0 ? "-" : "";
  const absolute = Math.abs(amount).toFixed(2);
  return `${sign}$${absolute}`;
}

export { formatCurrency };

// currency.test.ts
import { formatCurrency } from "./currency";

describe("formatCurrency", () => {
  it("formats a whole number with two decimal places", () => {
    expect(formatCurrency(100)).toBe("$100.00");
  });

  it("formats zero correctly", () => {
    expect(formatCurrency(0)).toBe("$0.00");
  });

  it("formats negative numbers with a leading minus sign", () => {
    expect(formatCurrency(-42.5)).toBe("-$42.50");
  });

  it("rounds to two decimal places", () => {
    expect(formatCurrency(19.999)).toBe("$20.00");
  });

  it("formats large values without truncating digits", () => {
    expect(formatCurrency(1234567.89)).toBe("$1234567.89");
  });
});

WHY THIS WORKS
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- Each `it()` covers one specific behavior (zero, negative, rounding, large
  values) rather than cramming multiple unrelated assertions into one test —
  when one fails, the test name alone tells you which case broke.
- Testing a pure function needs no mocks at all: same input always produces
  the same output, which is exactly what makes it fast and reliable to test.