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Error: Cannot find module 'express'

Exact error

Error: Cannot find module 'express'
Require stack: ...  code: 'MODULE_NOT_FOUND'

What it means

Node can't find a package (or local file) you required.

Likely causes

• Dependencies not installed in this environment.
• Package not listed in package.json.
• Wrong relative path for a local file.

How to fix it

1. Install and save the dependency:

   npm install express

2. For a local file, check the relative path, e.g. require('./routes/users') from the right directory.

Prevent this next time

• Commit package-lock.json or requirements.txt so every environment installs the same dependencies.
• Read the port from `process.env.PORT` and bind to 0.0.0.0 in hosted environments.

Guide: catch outages before your users do.

Related errors

• ModuleNotFoundError: No module named 'flask'
• Module not found: Can't resolve '...'