jestjs/jest · error · Error

Jest was unable to load the transformer defined for ${filena

Error message

Jest was unable to load the transformer defined for ${filename}. This is a bug in Jest, please open up an issue

What it means

Thrown by `_getTransformer` (ScriptTransformer.ts:336-338) when `_transformsAreLoaded` is true and a transform pattern matched the filename, but `_transformCache.get(transformCacheKey)` returns undefined. The contract is that `loadTransformers` populates the cache for every configured entry; a miss means the cache key computed at lookup differs from the key computed at load - an internal Jest consistency failure.

Source

Thrown at packages/jest-transform/src/ScriptTransformer.ts:336

      return null;
    }

    const transformPatternAndPath = this._getTransformPatternAndPath(filename);
    if (!Array.isArray(transformPatternAndPath)) {
      return null;
    }

    const [transformPattern, transformPath] = transformPatternAndPath;
    const transformCacheKey = this._buildTransformCacheKey(
      transformPattern,
      transformPath,
    );
    const transformer = this._transformCache.get(transformCacheKey);
    if (transformer !== undefined) {
      return transformer;
    }

    throw new Error(
      `Jest was unable to load the transformer defined for ${filename}. This is a bug in Jest, please open up an issue`,
    );
  }

  private _instrumentFile(
    filename: string,
    input: TransformedSource,
    canMapToInput: boolean,
    options: ReducedTransformOptions,
  ): TransformedSource {
    const inputCode = typeof input === 'string' ? input : input.code;
    const inputMap = typeof input === 'string' ? null : input.map;

    const result = babelTransform(inputCode, {
      auxiliaryCommentBefore: ' istanbul ignore next ',
      babelrc: false,
      caller: {
        name: '@jest/transform',

View on GitHub (pinned to f49721c78e)

Solutions

1. Confirm you are on a released Jest version (not a patched fork) and file the issue requested by the message if so.
2. If you monkey-patch `config.transform` or `_transformCache` at runtime, stop - the cache is populated once at load time.
3. Ensure `_buildTransformCacheKey` inputs (regex source, resolved transform path) are stable strings across calls.

// Internal consistency failure - catch, capture diagnostics, and report
try { transformer.transformSourceAsync(file, src); }
catch (e) {
  if (/unable to load the transformer.*bug in Jest/.test(e.message)) {
    captureState({ transform: config.transform, cache: [...transformer._transformCache.keys()] });
    throw new Error('file a Jest issue: transformer cache miss after load', { cause: e });
  }
  throw e;
}

When it happens

Trigger: Requires `_buildTransformCacheKey` (which mixes the transform-pattern regex source and the transformPath) to be non-deterministic between the load loop (ScriptTransformer.ts:297-301) and the lookup (ScriptTransformer.ts:327-330) - e.g. a regex `source` that varies, or a transformPath that resolves differently across calls.

Common situations: Effectively unreachable in stock Jest; seen with monkey-patches that mutate `_config.transform` after load, or with a custom fork that changes cache-key construction. The message itself directs users to file a Jest issue.

Related errors

• "cwd" must be an absolute path. cwd: ${cwd}
• ● Invalid transformer module: "${slash(transformPath)}" sp
• Jest: Transformers have not been loaded yet - make sure to r
• ● Invalid return value: `process()` or/and `processAsync()

AI-assisted analysis of jestjs/jest@f49721c78e (2026-08-03). Data as JSON: /data/errors/c8ef7a741edaaee6.json. Report an issue: GitHub.