jestjs/jest · error · Error

Missing data URI encoding

Error message

Missing data URI encoding

What it means

Thrown by parseDataUri when the MIME type is `application/wasm` but no encoding segment is present. WASM data URIs must be base64-encoded (binary cannot live in a URL-decoded text body), so Jest refuses to parse one without an explicit `;base64`.

Source

Thrown at packages/jest-runtime/src/internals/EsmLoader.ts:140

  return error;
}

// Decode a `data:` URI specifier into its mime type and decoded code/body.
// `application/wasm` returns a Buffer; everything else returns a UTF-8 string.
const dataURIRegex =
  /^data:(?<mime>text\/javascript|application\/json|application\/wasm)(?:;(?<encoding>charset=utf-8|base64))?,(?<code>.*)$/;

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

// Mirrors Node's `validateAttributes` in lib/internal/modules/esm/assert.js.
// The only deliberate divergence: missing `type: 'json'` warns instead of
// throwing — see the JSON branch below.
const warnedMissingJsonAttributePairs = new Set<string>();

View on GitHub (pinned to f49721c78e)

Solutions

1. Add `;base64` to the URI: `data:application/wasm;base64,<base64-encoded-bytes>`.
2. Generate the URI programmatically: `'data:application/wasm;base64,' + Buffer.from(wasmBytes).toString('base64')`.

Example fix

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

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

function buildWasmDataUri(bytes) {
  if (!bytes || !bytes.length) throw new Error('empty wasm bytes');
  return 'data:application/wasm;base64,' + Buffer.from(bytes).toString('base64');
}

function isBase64WasmDataUri(s: string): boolean {
  return /^data:application\/wasm;base64,/.test(s);
}

When it happens

Trigger: Importing a `data:application/wasm,...` URI without the `;base64` encoding token. WASM bodies are binary and cannot be represented as URL-encoded text, so the encoding is mandatory.

Common situations: Hand-writing a WASM data URI and forgetting the encoding; copying a JS data URI pattern and swapping only the MIME to wasm; a code generator that omits the encoding for wasm modules.

Related errors

• Invalid data URI encoding: ${encoding}
• Invalid data URI
• jest.config.mts requires native TypeScript support. Ensure y
• Unable to load resolver at ${options.resolver}
• Attempting to import a mock without a factory

AI-assisted analysis of jestjs/jest@f49721c78e (2026-08-03). Data as JSON: /data/errors/bd0b57076a6f91ef.json. Report an issue: GitHub.