vitest-dev/vitest · error · Error

Missing data URI encoding

Error message

Missing data URI encoding

What it means

Thrown by parseDataUri specifically for WebAssembly data URIs: when the MIME is `application/wasm` the encoding is mandatory (only base64 wasm is supported, because raw binary cannot live in a text URI). Reaching this branch means a wasm data URI had no `;base64` (or any encoding) segment.

Solutions

1. Encode the wasm as base64 and add `;base64`: `data:application/wasm;base64,<base64-bytes>`.
2. Load the wasm from a file URL instead of a data URI when feasible.
3. When generating URIs in code, always pair `application/wasm` with base64 encoding.

Example fix

// before
const url = `data:application/wasm,${rawBytes}`

// after
const url = `data:application/wasm;base64,${rawBytes.toString('base64')}`

function isBase64WasmDataUri(uri: string): boolean {
  return /^data:application\/wasm;base64,[A-Za-z0-9+/=]+$/.test(uri)
}

function isBase64WasmDataUri(uri: string): boolean {
  return /^data:application\/wasm;base64,[A-Za-z0-9+/=]+$/.test(uri)
}

When it happens

Trigger: Test code imports `data:application/wasm,<binary>` without an encoding, e.g. `await import('data:application/wasm,AGFzbQ...')`. Vitest requires `data:application/wasm;base64,...` so the bytes decode reliably.

Common situations: Generating wasm data URIs programmatically and forgetting the base64 encoding; copy-pasting a wasm data URI that omitted the encoding; tools that emit raw wasm bytes inline.

Related errors

• Invalid data URI encoding
• Invalid data URI
• Cannot import " ": its vm context was torn down.
• invalid diff config file
• invalid snapshot serializer file

AI-assisted analysis of vitest-dev/vitest@1fa9837ec2 (2026-08-11). Data as JSON: /api/errors/18b60fa34aff1b2d. Report an issue: GitHub.

Appendix: source

Thrown at packages/vitest/src/runtime/vm/esm-executor.ts:45

// `hasAsyncGraph` only exists on SourceTextModule — a SyntheticModule is
// synchronous by definition (its evaluation callback is sync)
function moduleHasAsyncGraph(module: VMModule): boolean {
  return module instanceof SourceTextModule && module.hasAsyncGraph()
}

const dataURIRegex
  = /^data:(?<mime>text\/javascript|application\/json|application\/wasm)(?:;(?<encoding>charset=utf-8|base64))?,(?<code>.*)$/

function parseDataUri(identifier: string): { mime: string; code: string | Buffer } {
  const match = identifier.match(dataURIRegex)
  if (!match || !match.groups) {
    throw new Error('Invalid data URI')
  }
  const { mime, encoding } = match.groups
  let code: string | Buffer = match.groups.code
  if (mime === 'application/wasm') {
    if (!encoding) {
      throw new Error('Missing data URI encoding')
    }
    if (encoding !== 'base64') {
      throw new Error(`Invalid data URI encoding: ${encoding}`)
    }
    return { mime, code: Buffer.from(code, 'base64') }
  }
  if (!encoding || encoding === 'charset=utf-8') {
    code = decodeURIComponent(code)
  }
  else if (encoding === 'base64') {
    code = Buffer.from(code, 'base64').toString()
  }
  else {
    throw new Error(`Invalid data URI encoding: ${encoding}`)
  }
  return { mime, code }
}

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