What is the purpose of installing dependencies (cd first-app && npm-install)?

Question

Node.js

Installed express, created the project express first-app,
The article says to install dependencies in this way cd first-app && npm-install .
I can’t understand what kind of addiction is and why it is needed.

Answer 1

Before learning Express, it would be nice to read what npm is and why it is needed. In short, without dependencies, nothing will work.
In more detail: npm install installs the packages specified in package.json in the node_modules directory. When writing in the application:
the node looks for the express package in the node_modules folder and loads it into the application.