Vitest Modern Testing (Advanced)

Configuration and Speed

Vitest reuses your Vite config, so aliases, plugins, and TypeScript transform settings apply to tests automatically:

// vitest.config.ts
import { defineConfig } from "vitest/config";

export default defineConfig({
  test: {
    environment: "node",       // or "jsdom" / "happy-dom"
    globals: true,
    restoreMocks: true,
  },
});

Tests run as native ESM in worker threads — no separate Babel/ts-jest pipeline — which is why watch mode and startup are fast. Vite resolve.alias covers path mappings for both app and tests, so tsconfig paths and test resolution cannot drift apart.

Mocking with vi

vi.mock is hoisted like Jest's, and its factory can reference vi.hoisted for values that must exist first:

const ids = vi.hoisted(() => ({ next: 1 }));
vi.mock("../src/db", () => ({
  insert: vi.fn(() => ({ id: ids.next++ })),
}));

import { insert } from "../src/db";
expect(insert({ a: 1 })).toEqual({ id: 1 });

Timers: vi.useFakeTimers() then vi.advanceTimersByTime(1000) drives debounce and interval logic without real waiting. vi.spyOn(obj, "method") keeps original behavior until you mockReturnValue, and expect(...).toHaveBeenCalled works because Vitest's assertion layer shares Jest-compatible matchers.

Pitfalls and Interview Angles

Specifier sensitivity. vi.mock("../src/db") must match how the test imports the module — same relative path, or mocking silently applies to a different resolved file.

Globals on versus off. With globals: false (the default) you import describe/it/expect from vitest; editor types then come from vitest/globals only when globals are enabled.

Coverage and type testing. coverage: { provider: "v8" } instruments without instrumenting transforms, and expectTypeOf<T>().toMatchTypeOf<U>() (or assertType) checks generics at compile time in the same file.

Interview: contrast Vitest with Jest architecturally — Vite's module graph and native ESM versus Jest's CommonJS-centric registry — and say which config keys must move when migrating.