Dependency

A dependency is an outside piece of software your project relies on to run, usually pulled in as a named package so you reuse that capability instead of writing it yourself.

A dependency is an outside piece of software your project relies on to run, usually pulled in as a named package so you reuse that capability instead of writing it yourself. The project keeps a list of these packages by name and version, and a tool installs them so the imported code is present wherever the project runs. Say you are building a to-do app and you want it to format dates as 'in 3 days' instead of a raw timestamp. Rather than write that calendar logic from scratch, you tell your assistant to add a date-formatting package; it gets added to the project's list, installed, and imported, and your code calls it directly.

Builder example

Dependencies decide whether your project runs the same on your machine and on the server that hosts it. If a package is missing from the project's list, the app works locally and then fails after deploy because the host never installed it. A package that changes its behavior in a new version can also break a feature that worked yesterday. Pinning each package to a known version, and asking your assistant to update one at a time, keeps a working app from breaking silently.

Common confusion: A dependency is code your project pulls in by name, while your own source files are code you wrote and maintain directly. Adding a package speeds up the build, and it also hands part of your app to an outside maintainer whose updates and security fixes you now depend on.