Node.js: 'Error: Cannot find module X' (code MODULE_NOT_FOUND) at runtime, even though it works on another machine. How do I fix it?

Cause: Node looked for the module and didn't find it. For a package name (require('express') / import 'express'), Node searches node_modules folders from the file's directory up to the filesystem root; for a path ('./utils'), it looks exactly there. Typical reasons: dependencies not installed (fresh clone, CI job or Docker image without npm install), the package is in devDependencies but you installed with --omit=dev / NODE_ENV=production, the package isn't in package.json at all (it worked locally only because it happened to be installed), a wrong relative path or letter case (Linux is case-sensitive, macOS and Windows usually aren't), a missing build step (the error names dist/index.js), or running from the wrong directory. Fix: 1. Read the error carefully: it says which module and which file required it ('Require stack'). A path like ./dist/... means a build output is missing; a package name means a dependency issue. 2. Install dependencies from the lockfile: npm ci (clean, exact) or npm install. In Docker, copy package.json and package-lock.json and run npm ci in the image, and don't copy a host node_modules folder over it (add node_modules to .dockerignore). 3. Make sure the package is declared: npm install X (runtime) or npm install -D X (dev only). If production needs it, it must be in dependencies, not devDependencies. 4. For relative paths, check exact file names and case, and the extension: in ES modules, relative imports need the extension ('./utils.js'). 5. If a build step is missing, run it (npm run build) before npm start, and make sure CI does too. 6. Corrupted installs: delete node_modules and reinstall with npm ci; native modules may also need rebuilding after a Node version change (npm rebuild). Verify: npm ls X shows the package, and node -e "require.resolve('X')" prints its path from the project directory. Sources: https://nodejs.org/api/modules.html https://docs.npmjs.com/cli/commands/npm-ci