Node ERR_MODULE_NOT_FOUND: module resolution and paths

Recognize the actual failure

An ECMAScript import resolves a package, a relative path or an absolute file URL according to Node's module resolution rules. Common causes are a missing package, a relative specifier without a required extension in ESM, a wrong filename case on Linux and imports from an unexpected working directory.

Inspect safely before changing anything

node --version
node -p "process.cwd()"
node -p "require.resolve('fs')"

Repair the cause rather than suppressing the warning

Inspect the exact specifier printed in the error, check package.json type/module mode, and test import('./lib/task.js') using the real case-sensitive path. npm install is appropriate only after confirming the dependency actually belongs in package.json; installing random packages by error-string search can introduce unnecessary risk.

Reference: https://nodejs.org/api/esm.html

Reproducible ESM fixture

Create a temporary folder containing the two files below. Run node app.mjs from that folder; no npm installation is needed.

// lib/math.mjs
export const double = n => n * 2;

// app.mjs
import { double } from './lib/math.mjs';
console.log(double(21));

The expected output is 42. Rename math.mjs to Math.mjs without updating the import: a case-sensitive filesystem will reject the reference. Restore the filename and change the import to ./lib/math: native ESM does not generally guess the missing extension for relative imports. The test distinguishes package resolution from a missing local file.

Decide which resolver is involved

An ERR_MODULE_NOT_FOUND stack originating from an import in .mjs or an ESM package is not automatically fixed by require.resolve(); CommonJS and ESM resolution differ. Print the exact attempted specifier, inspect package.json exports/type fields, and resolve relative paths against the importing module, not the shell's current directory. On Windows test case differences on a Linux CI worker too. Do not install an unknown similarly named package merely because the error contains its name.