Installing Node Packaged Modules

To use a Node module in your applications, it must first be installed where Node can find it. To install a Node module, use the npm install <module_name> command to download the Node module to your development environment and place it in the node_modules folder, where the install command is run. For example, the following command installs the express module:

npm install express

The output of the npm install command displays the HTTP requests to download the necessary files as well as the dependency hierarchy that was installed with the module. For example, Figure 3.3 shows part of the output from installing the express module.

Figure 3.3 Output from installing the express module.

Notice that several .tgz files are downloaded, decompressed, and installed. Figure 3.3 also shows the dependency hierarchy: You can see that express requires the methods, cookie-signature, range-parser, debug, buffer-crc32, fresh, cookie, mkdirp, commander, send, and connect modules. All these modules were downloaded during the install. Notice that the version of each dependency module is listed.

Node.js has to be able to handle dependency conflicts. For example, the express module requires cookie 0.1.0, but another module may require cookie 0.0.9. To handle this situation, a separate copy for the cookie module is placed in each module’s folder, under another node_modules folder.

To see how modules are stored in a hierarchy, consider the following example of how express looks on disk. Notice that the cookie and send modules are located under the express module hierarchy and that because the send module requires mime, it is located under the send hierarchy:

Click here to view code image

./
./node_modules
./node_modules/express
./node_modules/express/node_modules/cookie
./node_modules/express/node_modules/send
./node_modules/express/node_modules/send/node_modules/mime