TS2307: Cannot find module 'X' or its corresponding type declarations

Common causes

• The npm package isn't installed — run npm install to get it
• The @types package for the library isn't installed (e.g., @types/express for express)
• A relative import path is wrong or the file doesn't exist at that path
• The module's package.json doesn't include a types field and there's no DefinitelyTyped package

How to fix it

1. Install the package if missing: npm install package-name
2. Install type declarations: npm install --save-dev @types/package-name
3. Check the relative import path — make sure the file exists and the path is correct
4. If no @types package exists, add a declaration file: create module.d.ts with declare module 'module-name'

Example

Importing express before installing @types/express alongside the package