vitest-dev/vitest · error · Error

Data was not properly initialized. This is a bug in Vitest…

Error message

Data was not properly initialized. This is a bug in Vitest. Please, open a new issue with reproduction.

What it means

Thrown by `executeTests` when the module-level `preparedData` is still undefined. `preparedData` is populated by `prepareTestEnvironment` (which builds the runner, RPC, mocker, and worker state) before `executeTests` is called. If `executeTests` runs first, the runner/state/config are missing and execution cannot proceed. The message explicitly flags this as a Vitest internal bug rather than a user error.

Solutions

1. Use the standard Vitest browser tester entry point that calls `prepareTestEnvironment` before `executeTests`.
2. If customizing the tester, ensure `prepareTestEnvironment` has resolved before invoking `executeTests`.
3. Treat as a Vitest bug: open an issue with a minimal reproduction.

// internal contract: prepareTestEnvironment must run first
if (!preparedData) {
  await prepareTestEnvironment(opts)
}
await executeTests('run', specs)

When it happens

Trigger: The browser tester's bootstrap sequence is interrupted or reordered so `executeTests('run'|'collect', specs)` is called before `prepareTestEnvironment` completes; a custom entry point that calls `executeTests` directly.

Common situations: An upstream Vitest regression in the tester bootstrap; a race where the runner message arrives before environment preparation; a customized tester HTML/entry that skips `prepareTestEnvironment`.

Related errors

• Worker state is not found. This is an issue with Vitest…
• Browser is not initialized
• Cannot find iframe element. This is a bug in Vitest…
• Don't have access to the "vitest" instance yet. This is a…
• Expected DOM element to be an instance of Element, received

AI-assisted analysis of vitest-dev/vitest@1fa9837ec2 (2026-08-11). Data as JSON: /api/errors/3c7dbbcdeac650bc. Report an issue: GitHub.

Appendix: source

Thrown at packages/browser/src/client/tester/tester.ts:229

    })
  }

  state.durations.prepare = performance.now() - options.startTime

  return {
    runner,
    config,
    state,
  }
}

let preparedData:
  | Awaited<ReturnType<typeof prepareTestEnvironment>>
  | undefined

async function executeTests(method: 'run' | 'collect', specifications: FileSpecification[]) {
  if (!preparedData) {
    throw new Error(`Data was not properly initialized. This is a bug in Vitest. Please, open a new issue with reproduction.`)
  }

  debug?.('runner resolved successfully')

  const { runner, state } = preparedData

  state.ctx.files = specifications
  runner.setMethod(method)

  const version = url.searchParams.get('browserv') || ''
  specifications.forEach(({ filepath }) => {
    const currentVersion = browserHashMap.get(filepath)
    if (!currentVersion || currentVersion[1] !== version) {
      browserHashMap.set(filepath, version)
    }
  })

  for (const file of specifications) {

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