One of the key steps in becoming immersed in an ecosystem is learning its vocabulary. Node.js and npm have very specific definitions of packages and modules, which are easy to mix up. We'll discuss those definitions here, make them distinct, and explain why certain default files are named the way they are. ## tl;dr * A package is a file or directory that is described by a package.json. This can happen in a bunch of different ways! For more info, see "What is a package?, below. * A module is any file or directory that can be loaded by Node.js' require(). Again, there are several configurations that allow this to happen. For more info, see "What is a module?", below. ## What is a package? A package is any of: * a) a folder containing a program described by a package.json file * b) a gzipped tarball containing (a) * c) a url that resolves to (b) * d) a <name>@<version> that is published on the registry with (c) * e) a <name>@<tag> that points to (d) * f) a <name> that has a latest tag satisfying (e) * g) a git url that, when cloned, results in (a). Noting all these package possibilities, it follows that even if you never publish your package to the public registry, you can still get a lot of benefits of using npm: * if you just want to write a node program, and/or * if you also want to be able to easily install it elsewhere after packing it up into a tarball Git urls can be of the form: ~~~ git://github.com/user/project.git#commit-ish git+ssh://user@hostname:project.git#commit-ish git+http://user@hostname/project/blah.git#commit-ish git+https://user@hostname/project/blah.git#commit-ish ~~~ The commit-ish can be any tag, sha, or branch which can be supplied as an argument to git checkout. The default is master. ## What is a module? A module is anything that can be loaded with require() in a Node.js program. The following are all examples of things that can be loaded as modules: * A folder with a package.json file containing a main field. * A folder with an index.js file in it. * A JavaScript file. ## Most npm packages are modules Generally, npm packages that are used in Node.js program are loaded with require, making them modules. However, there's no requirement that an npm package be a module! Some packages, e.g., cli packages only contain an executable command-line interface, and don't provide a main field for use in Node.js programs. These packages are not modules. Almost all npm packages (at least, those that are Node programs) contain many modules within them (because every file they load with require() is a module). In the context of a Node program, the module is also the thing that was loaded from a file. For example, in the following program: var req = require('request') we might say that "The variable req refers to the request module". ## File and Directory Names in the Node.js and npm Ecosystem So, why is it the node_modules folder, but package.json file? Why not node_packages or module.json? The package.json file defines the package. (See "What is a package?", above.) The node_modules folder is the place Node.js looks for modules. (See "What is a module?", above.) For example, if you create a file at node_modules/foo.js and then had a program that did var f = require('foo.js'), it would load the module. However, foo.js is not a "package" in this case, because it does not have a package.json. Alternatively, if you create a package which does not have an index.js or a "main" field in the package.json file, then it is not a module. Even if it's installed in node_modules, it can't be an argument to require().