Testing & Code Quality

Vitest Unit Tests

Vitest is a Vite-native test runner for JavaScript and TypeScript projects, supporting fast unit tests, mocks, fake timers, coverage, browser mode, and familiar Jest-like APIs.

Unit tests should verify behavior, not implementation trivia
Fast tests shape developer behavior
Vitest fits Vite projects naturally
Coverage is a signal, not a goal
Test boundaries should be chosen deliberately

Simple behavior test

import { describe, expect, it } from "vitest"
import { parseUserId } from "./user-id"

describe("parseUserId", () => {
  it("accepts valid user ids", () => {
    expect(parseUserId("user_123")).toBe("user_123")
  })

  it("rejects invalid ids", () => {
    expect(() => parseUserId("product_123")).toThrow("Invalid")
  })
})

One happy path, one failure path, both tied to observable behavior.

Good unit-test targets
Target	Why
Pure functions	Deterministic and fast
Parsers/validators	Boundary safety
Reducers/state machines	Many state transitions
API wrappers	Error/status handling
Formatting/mapping logic	Easy regressions