TypeScript error: Cannot find module 'X' or its corresponding type declarations

What causes this error

This error covers two genuinely different problems that look identical: the package itself isn't installed, or the package exists but ships no type information and there's no separate @types package for it either.

The Fix

# 1. Confirm the package is actually installed
npm ls some-package

# 2. If installed but still erroring, try the matching types package
npm install -D @types/some-package

# 3. If no @types package exists, declare it yourself
# Create a file like src/types/some-package.d.ts:
declare module 'some-package';

Common causes / variations

• Forgetting to run npm install after pulling changes that added a new dependency
• A package written in plain JS with no published types and no community @types package
• Path alias imports (@/lib/foo) not resolving because tsconfig.json's paths is misconfigured

Related errors

See also: Property does not exist on type 'never'.