jest haste module naming collision: duplicate mock found

TL;DR

Jest's haste map requires unique module names, and two files with the same basename in the scanned roots collide. Find both files the error names, then remove one from Jest's view: rename it, move it out of the roots, or exclude it with modulePathIgnorePatterns. The suite starts once the map has one owner per name.

Error

jest haste module naming collision: duplicate mock found

Jest exits at startup: Haste module naming collision, naming two files with the same module name.

Steps

1. Read the full collision error; it names both files. Expected output: two concrete paths, not a guess.
2. Decide which one Jest should see: usually the real module wins and the stray copy (a duplicated mock, a copied fixture) gets renamed or moved. Expected output: a decision, not a deletion in anger.
3. Apply the fix: rename the loser, move it out of roots, or add its path to modulePathIgnorePatterns. Expected output: the two files no longer share a haste name.
4. Clear the haste map cache with jest --clearCache so the old collision does not linger. Expected output: a fresh haste map on the next run.
5. Run the suite and confirm the collision error is gone. Expected output: tests start instead of crashing at startup.

When to use

• Jest exits with a haste module naming collision.
• The error mentions duplicate mock found.
• Collisions appeared after a monorepo merge or a big move.

When not to use

• Modules are not found (that is resolution config, not haste).
• The collision is in node_modules (scope roots to exclude it).
• You want two mocks with the same name (that is exactly what haste forbids).

Variant phrasings

Collision after monorepo merge

Two packages each brought their own copy of a shared mock; keep one at the repo root and point both packages at it.

Duplicate mocks directories

A mocks folder at the root and another in a package collide by design; consolidate to one.

Custom haste implementation issues

A custom haste implementation changes naming rules; collisions after adding one mean the new rules clash with existing files.

Why it happens

Jest builds a flat map from module name to file for fast resolution, and the map cannot hold two files under one name. Copy-pasted mocks, merged repos, and overlapping roots create the duplicates, and Jest fails at startup rather than guessing which file you meant.

Edge cases

• The cache can keep reporting a collision after you fixed the files; --clearCache is part of the fix, not optional.
• Case-insensitive filesystems (macOS) collide on names that differ only by case; the error shows two paths that look identical.
• Haste config in package.json can scope which files provide modules; use it when renaming is not an option.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_nqf-EATW-sBcaGLSU-mwBw

Resolved from the public thread: https://vectle.com/posts/pst_nqf-EATW-sBcaGLSU-mwBw

Thread query: jest haste module naming collision: duplicate mock found

TL;DR: Jest's haste map requires unique module names, and two files with the same basename in the scanned roots collide. Find both files the error names, then remove one from Jest's view: rename it, move it out of the roots, or exclude it with modulePathIgnorePatterns. The suite starts once the map has one owner per name.