jestjs/jest · error · DuplicateHasteCandidatesError

The name `${name}` was looked up in the Haste module map. It

Error message

The name `${name}` was looked up in the Haste module map. It cannot be resolved, because there exists several different files, or packages, that provide a module for that particular name and platform. ${platformMessage} You must delete or exclude files until there remains only one of these:

  * `${dupFilePath}` (${getTypeMessage(dupFileType)})

What it means

jest-haste-map builds a haste-id → file map for platforms/generic resolution. When a single haste name resolves to more than one file for the same platform, `_assertNoDuplicates` throws DuplicateHasteCandidatesError (ModuleMap.ts:191) listing every conflicting path, because the resolver cannot pick one deterministically. This is the classic Jest 'duplicate manual mocks / haste collision' failure.

Source

Thrown at packages/jest-haste-map/src/ModuleMap.ts:191

  private _assertNoDuplicates(
    name: string,
    platform: string,
    supportsNativePlatform: boolean,
    relativePathSet: DuplicatesSet | null,
  ) {
    if (relativePathSet == null) {
      return;
    }
    // Force flow refinement
    const previousSet = relativePathSet;
    const duplicates = new Map();

    for (const [relativePath, type] of previousSet) {
      const duplicatePath = fastPath.resolve(this._raw.rootDir, relativePath);
      duplicates.set(duplicatePath, type);
    }

    throw new DuplicateHasteCandidatesError(
      name,
      platform,
      supportsNativePlatform,
      duplicates,
    );
  }

  static create(rootDir: string): ModuleMap {
    return new ModuleMap({
      duplicates: new Map(),
      map: new Map(),
      mocks: new Map(),
      rootDir,
    });
  }
}

class DuplicateHasteCandidatesError extends Error {

View on GitHub (pinned to f49721c78e)

Solutions

1. Read the listed duplicate paths and delete or rename one of them.
2. Add `modulePathIgnorePatterns` / ` haste.providesModuleNodeModules` / `roots` config so only one copy is crawled.
3. If a manual mock collides with a real package, remove the `__mocks__/<name>.js` entry or scope it via configuration.
4. For monorepos, ensure each package's `__mocks__` is not globally visible; use `projects`/workspaces config to isolate.
5. If a symlink is the cause, dedupe with yarn `resolutions` or npm `overrides`.

Example fix

// before: both __mocks__/db.js and node_modules/db exist
testEnvironment: 'node'

// after: remove the global manual mock and scope it
// delete __mocks__/db.js, instead mock at test level:
jest.mock('db', () => ({ query: jest.fn() }));

// preflight: scan for haste name collisions before running
const seen = new Map();
for (const f of crawl('src')) {
  const name = readProvidesModuleName(f);
  if (name && seen.has(name)) throw new Error(`duplicate haste id ${name}: ${seen.get(name)} vs ${f}`);
  if (name) seen.set(name, f);
}

const hasNoHasteCollision = (map: Map<string, string>) => map.size === new Set(map.values()).size;

When it happens

Trigger: Two files declare the same haste module name (e.g. `@providesModule Foo`) for the same platform; a manual mock in `__mocks__` collides with a real module of the same name; an npm dependency and a local file share a haste id; platform-specific files (`.ios.js`/`.android.js`) collide on the generic platform.

Common situations: Adding a manual mock that duplicates an installed package name; monorepo with two packages providing the same module; symlinking node_modules so a package is crawled twice; leftover generated files sharing a name; RN-style haste modules colliding after a dependency addition.

Related errors

• Cannot find module '${presetPath}'
• Could not resolve a module for a custom reporter. Module n
• haste.enableSymlinks is incompatible with watchman
• Duplicated files or mocks. Please check the console for more
• Cannot resolve module '${moduleName}' from paths ['${paths}'

AI-assisted analysis of jestjs/jest@f49721c78e (2026-08-03). Data as JSON: /data/errors/4f5ea2458c0761e1.json. Report an issue: GitHub.