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node_modules

Where installed packages live, and why it gets so big.

node_modules is the folder where your package manager installs the libraries your project depends on. Running npm install reads package.json and the lock file, then fills the folder with those packages.

npm install   # creates or updates node_modules to match the lock file

It gets large because packages depend on other packages, and each of those brings its own. A small project can end up with thousands of files in here. You almost never need to edit this folder, so treat it as a generated output.

Three habits keep it manageable:

• Don’t commit node_modules to git. Add it to .gitignore.
• Do commit the lock file (such as package-lock.json), so everyone and your deployment install the same versions.
• When it’s broken, delete it and reinstall. That’s usually faster than hunting for the cause.

The classic mistake is copying node_modules between machines. Some packages contain compiled code for one operating system or one Node version, so a copy from elsewhere can fail in confusing ways. Install on each machine instead. Python projects have a similar role for virtual environments, often stored in a .venv folder.