vitest-dev/vitest · error · Error

Missing data URI encoding

Error message

Missing data URI encoding

What it means

Thrown by createDataModule when a `data:application/wasm,...` URI is imported but has no encoding segment. WebAssembly data URIs require base64 encoding (raw binary wasm cannot be embedded as URI-encoded text safely), so the wasm branch insists on an explicit `;base64`.

Source

Thrown at packages/vitest/src/runtime/vm/esm-executor.ts:212

  public async createDataModule(identifier: string): Promise<VMModule> {
    const cached = this.moduleCache.get(identifier)
    if (cached) {
      return cached
    }

    const match = identifier.match(dataURIRegex)

    if (!match || !match.groups) {
      throw new Error('Invalid data URI')
    }

    const mime = match.groups.mime
    const encoding = match.groups.encoding

    if (mime === 'application/wasm') {
      if (!encoding) {
        throw new Error('Missing data URI encoding')
      }

      if (encoding !== 'base64') {
        throw new Error(`Invalid data URI encoding: ${encoding}`)
      }

      const module = this.loadWebAssemblyModule(
        Buffer.from(match.groups.code, 'base64'),
        identifier,
      )
      this.moduleCache.set(identifier, module)
      return module
    }

    let code = match.groups.code
    if (!encoding || encoding === 'charset=utf-8') {
      code = decodeURIComponent(code)
    }

View on GitHub (pinned to d568f8ce37)

Solutions

1. Add `;base64` to the wasm data URI and base64-encode the binary payload.
2. Load the wasm from a file instead: import the bytes and use WebAssembly.compile, or use createWebAssemblyModule with a Buffer.
3. Generate the data URI with a library that always emits the encoding segment.

Example fix

// before
import wasm from 'data:application/wasm,AGFzbQ...'

// after
import wasm from 'data:application/wasm;base64,AGFzbQ=='

function assertWasmDataUri(uri: string) {
  const m = uri.match(/^data:application\/wasm;([^,]+),/)
  if (m && m[1] !== 'base64') throw new Error('wasm data URI must use ;base64')
}

When it happens

Trigger: Importing `data:application/wasm,<bytes>` without the `;base64` encoding marker. The wasm code path runs only for the application/wasm mime, and the encoding group is empty/undefined.

Common situations: Hand-constructing a wasm data URI and forgetting the encoding. Tooling that emits data URIs without the encoding field for binary payloads.

Related errors

• Invalid data URI encoding: ${encoding}
• Invalid data URI
• import of '${fileUrl}' by undefined is not supported: http c
• Expected IP address, received ${address}
• Cannot parse the module format of '${url}' because "module.f

AI-assisted analysis of vitest-dev/vitest@d568f8ce37 (2026-08-03). Data as JSON: /data/errors/18b60fa34aff1b2d.json. Report an issue: GitHub.