Fix "Cannot find module" with ES modules in Node.js

Node.js ES modules require explicit file extensions in import paths. Unlike CommonJS (require), the module resolver does not try .js, .ts, or /index.js automatically.

The Fix

Add the file extension to your import:

// Wrong
import { handler } from "./routes/auth";

// Correct
import { handler } from "./routes/auth.js";

Yes, use .js even if your source file is .ts. TypeScript compiles .ts to .js, and Node resolves imports at runtime against the compiled output.

Root cause

When "type": "module" is set in package.json, Node uses the ESM resolver. The ESM spec requires explicit extensions, unlike CommonJS which probes multiple extensions.

Also check

1. package.json has "type": "module" if you’re using import/export syntax
2. tsconfig.json has "moduleResolution": "nodenext" or "bundler" for proper TypeScript ESM support
3. Directory imports need /index.js: import { foo } from "./utils/index.js", not "./utils"

If you’re using a bundler

Bundlers (Vite, esbuild, webpack) resolve imports themselves and don’t need file extensions. This error only happens when Node.js resolves imports directly (server-side code, scripts, tests without a bundler).