Dependency

An external library or package that your project relies on. Listed in package.json, installed with npm. Examples: React, Stripe's SDK, a date formatting library.

Dependencies let you use pre-built solutions for common problems, but they also introduce risk (security vulnerabilities, breaking changes, abandonment). Keeping them updated is part of maintenance.

A founder's app has 40 dependencies. Her developer runs npm audit weekly to check for security vulnerabilities and npm update to keep them current.