jestjs/jest · error · Error

${callerName} cannot be nested inside another ${callerName}

Error message

${callerName} cannot be nested inside another ${callerName} or ${other}.

What it means

Thrown by `ModuleRegistries.enterIsolated` when `jest.isolateModules` (or `isolateModulesAsync`) is invoked while another isolation scope is already active. Jest's module registries support a single layer of isolation overlay (ModuleRegistries.ts:143-153); nesting would silently corrupt module resolution so the runtime hard-fails instead.

Source

Thrown at packages/jest-runtime/src/internals/ModuleRegistries.ts:149

  getInternalCjsRegistry(): ModuleRegistry {
    return this.internalModuleRegistry;
  }

  getActiveMockRegistry(): Map<string, unknown> {
    return this.isolation?.mock ?? this.mockRegistry;
  }

  isIsolated(): boolean {
    return this.isolation !== null;
  }

  enterIsolated(callerName: 'isolateModules' | 'isolateModulesAsync'): void {
    if (this.isIsolated()) {
      const other =
        callerName === 'isolateModules'
          ? 'isolateModulesAsync'
          : 'isolateModules';
      throw new Error(
        `${callerName} cannot be nested inside another ${callerName} or ${other}.`,
      );
    }
    this.isolation = new Isolation();
  }

  exitIsolated(): void {
    this.isolation?.clear();
    this.isolation = null;
  }

  // Loads `fn` against fresh CJS + mock registries, then restores the
  // originals. Used by `_generateMock` to keep automock loading from
  // polluting the real caches.
  withScratchRegistries<T>(fn: () => T): T {
    const originalMock = this.mockRegistry;
    const originalModule = this.moduleRegistry;
    this.mockRegistry = new Map();

View on GitHub (pinned to f49721c78e)

Solutions

Flatten the nesting: keep a single `jest.isolateModules` (or `isolateModulesAsync`) call and do all the per-module `require` work inside it.
If you need two distinct module states, run them sequentially in separate `isolateModules` blocks rather than nested.
Audit helper functions for hidden `isolateModules` wrappers and remove the inner one.
Switch the outer call to `jest.isolateModulesAsync` if you need `await` inside, and don't re-enter from the callback.

Example fix

// before
jest.isolateModules(() => {
  const a = require('mod');
  jest.isolateModules(() => {
    const b = require('mod'); // throws
  });
});

// after — sequential, not nested
jest.isolateModules(() => { const a = require('mod'); });
jest.isolateModules(() => { const b = require('mod'); });

Defensive patterns

Strategy: validation

Validate before calling

let isolationDepth = 0;
function safeIsolate(fn: () => void) {
  if (isolationDepth > 0) {
    throw new Error('Refusing to nest isolateModules');
  }
  isolationDepth++;
  try {
    jest.isolateModules(fn);
  } finally {
    isolationDepth--;
  }
}

Prevention

Centralize all isolateModules usage behind a single helper so nested calls are caught at the helper boundary.
Never put isolateModules inside a helper that itself is called from inside isolateModules.
Prefer sequential isolated blocks over nested ones when you need multiple module states.

When it happens

Trigger: Calling `jest.isolateModules(() => { jest.isolateModules(() => { ... }) })`. Also when an `isolateModulesAsync` callback body awaits code that itself calls `isolateModules`, or vice versa (the message names both).

Common situations: Test helpers that wrap `require` in `isolateModules` invoked from inside another test helper already wrapping in `isolateModules`. Refactoring shared setup without realizing both layers isolate.

Related errors

Tests cannot be nested. Test "${spec.description}" cannot ru

AI-assisted analysis of jestjs/jest@f49721c78e (2026-08-03). Data as JSON: /data/errors/cf8a7d71d72f69e4.json. Report an issue: GitHub.