Tests on the core logic

Test the functions that hold your project's rules, not the file or network code around them. Vitest is a common choice for unit tests. Install it with npm install -D vitest, name files like budget.test.js, and write tests such as test("totals a month", () => { expect(monthlyTotal(entries)).toBe(42.5); }).

Node.js also ships a built-in runner, node --test with node:test and node:assert, if you want no dependency at all. Cover edge cases such as an empty array, one entry and invalid input. Use expect(() => parseAmount("abc")).toThrow() for rejected input, and toBeCloseTo for decimals.

Put the runner in the test script so npm test works. When you find a bug, write the failing test first.