NestJS jest "Cannot find module '@app/...'" path alias (moduleNameMapper) in CI

jest resolves modules with Node resolution, which does not understand tsconfig paths. An alias like @app/users that tsc accepts fails in jest unless you map it with moduleNameMapper.

What this error means

Unit tests fail with "Cannot find module '@app/users' from 'src/...'" even though the same import compiles fine with tsc and nest build.

NestJS

Cannot find module '@app/users/users.service' or its corresponding type declarations.
      at Resolver._throwModNotFoundError (node_modules/jest-resolve/build/resolver.js:...)

Common causes

jest does not read tsconfig paths

tsc maps @app/* via paths, but jest uses its own resolver and ignores tsconfig, so the alias is unknown.

moduleNameMapper is missing or out of sync

No mapping (or a stale one) means aliases added to tsconfig were never mirrored into the jest config.

How to fix it

Generate the mapper from tsconfig

Use ts-jest's helper to build moduleNameMapper from tsconfig paths.
Set the rootDir and prefix so mapped paths resolve.
Re-run jest so aliases match tsc.

jest.config.js

const { pathsToModuleNameMapper } = require('ts-jest');
const { compilerOptions } = require('./tsconfig.json');
module.exports = {
  moduleNameMapper: pathsToModuleNameMapper(compilerOptions.paths, { prefix: '<rootDir>/' }),
};

Map aliases explicitly

For the default Nest jest block in package.json, add an explicit mapping.

package.json

"moduleNameMapper": { "^@app/(.*)$": "<rootDir>/$1" }

How to prevent it

Generate moduleNameMapper from tsconfig so aliases never drift.
Keep one source of truth for path aliases (tsconfig) and derive the rest.
Run the jest suite in CI so alias breakage is caught immediately.