vitest-dev/vitest · error · Error

[vitest] `importScripts` is not supported in Vite workers…

Error message

[vitest] `importScripts` is not supported in Vite workers. Please, consider using `import` instead.

What it means

@vitest/web-worker runs workers as ES modules through Vite, so the classic Web Worker importScripts(...) API is fundamentally incompatible. The runner injects a replacement importScripts (runner.ts:39-43) into the worker context that throws immediately, surfacing the incompatibility at the call site instead of failing silently.

Solutions

Replace importScripts(url) with a dynamic ESM import: await import(url).
Convert the classic script to an ES module (add export/import statements) and use a static import.
If you only need side effects, use vi.mock or a static import of the module instead of importScripts.

Example fix

// before (inside the worker)
importScripts('https://cdn.example/lib.js')

// after
await import('https://cdn.example/lib.js')
// or for local modules: import './lib.js'

Defensive patterns

Strategy: type-guard

Validate before calling

// Scan worker source before registering it as a @vitest/web-worker.
function workerUsesImportScripts(source) {
  return /\bimportScripts\s*\(/.test(source)
}
if (workerUsesImportScripts(workerSource)) {
  throw new Error('Worker source uses importScripts; migrate to dynamic import().')
}

Type guard

function isEsmWorkerSource(src: string): boolean {
  return !/\bimportScripts\s*\(/.test(src)
}

Try / catch

// inside the worker module, isolate the legacy call
try {
  // @ts-expect-error legacy global
  importScripts(url)
} catch (e) {
  if (e instanceof Error && /importScripts is not supported/.test(e.message)) {
    console.warn('Blocked importScripts; migrate this call to: await import(url)')
  } else {
    throw e
  }
}

Prevention

Migrate worker code to ESM (static or dynamic import) before running under @vitest/web-worker.
Audit third-party worker libs for importScripts usage before pulling them in.
Add a lint rule or pre-test grep for importScripts in worker sources.
Prefer static imports where possible; they surface load failures at build time.

When it happens

Trigger: Code executed inside a @vitest/web-worker Worker calling importScripts(...) at runtime. The throw fires on the call, inside the worker realm.

Common situations: Porting a library built for classic workers; legacy code that still uses importScripts; a third-party script pulled into the worker that calls importScripts; code not yet migrated to ESM.

Related errors

AI-assisted analysis of vitest-dev/vitest@1fa9837ec2 (2026-08-11). Data as JSON: /api/errors/d33d5e644c8abaae. Report an issue: GitHub.

Appendix: source

Thrown at packages/web-worker/src/runner.ts:40

  const evaluator = new VitestModuleEvaluator(vm, {
    interopDefault: state.config.deps.interopDefault,
    injectCjsGlobals: state.config.injectCjsGlobals,
    moduleExecutionInfo: state.moduleExecutionInfo,
    getCurrentTestFilepath: () => state.filepath,
    compiledFunctionArgumentsNames,
    compiledFunctionArgumentsValues,
  })

  return startVitestModuleRunner({
    evaluator,
    evaluatedModules: state.evaluatedModules,
    mocker,
    state,
  })
}

function importScripts() {
  throw new Error(
    '[vitest] `importScripts` is not supported in Vite workers. Please, consider using `import` instead.',
  )
}

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