Why vi.stubEnv Doesn't Work for import.meta.env.DEV

By Jorge Saud · April 28, 2026

This one cost me a debugging session. Short post, but worth writing down.

The Problem

You have code that branches on import.meta.env.DEV:

export function isDraftInDevMode(data: { draft?: boolean }): boolean {
  return import.meta.env.DEV && data.draft === true
}

You want to test both branches, so you reach for vi.stubEnv:

it('returns false in production', () => {
  vi.stubEnv('DEV', 'false')  // ← looks right
  expect(isDraftInDevMode({ draft: true })).toBe(false)
})

The test passes in CI. It passes locally. But the assertion is a lie — the function is actually returning true.

Why

vi.stubEnv works by setting values on process.env (and import.meta.env for Vite’s env surface). The values are always strings — that’s the contract of environment variables.

Vite’s DEV flag is special. At build time, Vite replaces import.meta.env.DEV with the literal boolean true or false. But at test time in Vitest, that replacement doesn’t happen — import.meta.env is a mutable object and DEV stays as whatever type it was initialized with.

So when you call vi.stubEnv('DEV', 'false'), you set import.meta.env.DEV to the string 'false'. And the string 'false' is truthy in JavaScript. Your production-mode branch never actually runs.

vi.stubEnv('DEV', 'false')
console.log(typeof import.meta.env.DEV)  // "string"
console.log(!!import.meta.env.DEV)       // true — 'false' is truthy!

The Fix

Skip vi.stubEnv for boolean env flags and assign directly:

import.meta.env.DEV = false  // production mode
import.meta.env.DEV = true   // dev mode

In Vitest’s test environment, import.meta.env is a plain mutable object. Direct assignment works exactly as expected:

it('returns false in production', () => {
  import.meta.env.DEV = false
  expect(isDraftInDevMode({ draft: true })).toBe(false)  // actually false now
})

it('returns true in dev with draft=true', () => {
  import.meta.env.DEV = true
  expect(isDraftInDevMode({ draft: true })).toBe(true)
})

Cleanup

If you’re setting import.meta.env.DEV across multiple tests, reset it in afterEach so tests don’t leak state:

const originalDEV = import.meta.env.DEV

afterEach(() => {
  import.meta.env.DEV = originalDEV
})

Or if your whole describe block is testing production behavior, set it once in beforeAll:

describe('production mode', () => {
  beforeAll(() => { import.meta.env.DEV = false })
  afterAll(() => { import.meta.env.DEV = true })

  it('filters drafts', () => { /* ... */ })
  it('filters future dates', () => { /* ... */ })
})

When vi.stubEnv Is the Right Tool

vi.stubEnv is correct for string env vars — RESEND_API_KEY, POSTGRES_URL, anything that comes from .env files. Those are always strings and vi.stubEnv handles the restore-on-cleanup lifecycle cleanly.

It’s only DEV, PROD, SSR, and MODE that have this problem — the Vite-specific boolean/string flags that get statically replaced at build time but remain mutable objects in tests.