Claude Code Jest Test Runner Mock (2026)

The Error

FAIL src/__tests__/api.test.ts
  ● API Client › should call endpoint
    TypeError: Cannot read properties of undefined (reading 'mockImplementation')
      at Object.<anonymous> (src/__tests__/api.test.ts:15:22)
# Or:
jest.mock() calls must be at the top level of the module.
  Cannot call jest.mock() inside a function, test, or describe block.
# Or:
The module factory of `jest.mock()` is not allowed to reference
  any out-of-scope variables.

The Fix

  1. Move jest.mock() to the top level and use factory functions
// WRONG — mock inside describe
describe("API", () => {
  jest.mock("./api-client");  // Error: not top level
// CORRECT — mock at module top level
jest.mock("./api-client", () => ({
  fetchData: jest.fn(),
}));
import { fetchData } from "./api-client";
describe("API", () => {
  beforeEach(() => {
    jest.clearAllMocks();  // Reset between tests
  });
  it("should call endpoint", () => {
    (fetchData as jest.Mock).mockResolvedValue({ data: "test" });
    // ... test logic
  });
});
  1. For Claude-generated tests, fix the mock variable scoping
// WRONG — referencing outer variable in mock factory
const mockFn = jest.fn();
jest.mock("./module", () => ({ handler: mockFn }));
// Error: mockFn is out of scope
// CORRECT — use inline mock or jest.mocked()
jest.mock("./module");
import { handler } from "./module";
it("works", () => {
  const mocked = jest.mocked(handler);
  mocked.mockReturnValue("test");
});
  1. Verify the fix:
npx jest --testPathPattern="api.test" --verbose
# Expected: PASS with all tests green

Why This Happens

Jest hoists jest.mock() calls to the top of the file during compilation, before any imports execute. This hoisting means mock factories cannot reference variables declared after the mock call — even though they appear before it in the source code. Claude Code sometimes generates mocks inside describe or it blocks, or creates factory functions that reference variables defined between the mock call and the import, both of which break Jest’s hoisting semantics.

If That Doesn’t Work

  • Alternative 1: Use jest.spyOn() instead of jest.mock() for targeted method mocking without hoisting issues: jest.spyOn(module, 'method').mockReturnValue('test')
  • Alternative 2: Use manual mocks in __mocks__/ directories which Jest discovers automatically
  • Check: Run npx jest --showConfig | grep transform to verify your transform pipeline — TypeScript transforms can interfere with mock hoisting

Prevention

Add to your CLAUDE.md:

Place all jest.mock() calls at the file's top level, before imports. Use jest.mocked() for type-safe mock access. Call jest.clearAllMocks() in beforeEach. Never reference outer-scope variables in mock factory functions.

Related articles: Jest Unit Testing Guide, Claude Code Breaks Tests Fix, Debugging Skills

See Also

This fix also applies if you see variations of this error:

  • Connection or process errors with similar root causes in the same subsystem
  • Timeout variants where the operation starts but does not complete
  • Permission variants where access is denied to the same resource
  • Configuration variants where the same setting is missing or malformed

If your specific error message differs slightly from the one shown above, the fix is likely the same. The key indicator is the operation that failed (shown in the stack trace) rather than the exact wording of the message.

Frequently Asked Questions

Does this error affect all operating systems?

This error can occur on macOS, Linux, and Windows (WSL). The exact error message may differ slightly between platforms, but the root cause and fix are the same. macOS users may see additional Gatekeeper or notarization prompts. Linux users should check that the relevant system packages are installed. Windows users should ensure they are running inside WSL2, not native Windows.

Will this error come back after updating Claude Code?

Updates can occasionally reintroduce this error if the update changes default configurations or dependency requirements. After updating Claude Code, verify your project still builds and runs correctly. If the error returns, reapply the fix and check the changelog for breaking changes.

Can this error cause data loss?

No, this error occurs before or during an operation and does not corrupt existing files. Claude Code’s edit operations are atomic — they either complete fully or not at all. However, if the error occurs during a multi-step operation, you may have partial changes that need to be reviewed with git diff before continuing.

How do I report this error to Anthropic if the fix does not work?

Open an issue at github.com/anthropics/claude-code with: (1) the full error message including stack trace, (2) your Node.js version (node --version), (3) your Claude Code version (claude --version), (4) your operating system and version, and (5) the command or operation that triggered the error.

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