'express' is not executable

This is quite basic. Many examples show terminal commands such as: 'express name-of-directory-to-be-created'. Yet on my system, though I've installed node.js (and command: node -v works, and npm -v works), command: express fails with: command not found Further investigation shows that, on my macbook air, all incidences of express.js are not executable. I believe express was installed using npm express or npm -g express. Please, what is going on? Are the examples correct? ( There are several, all from various sources. ) How may I execute

express directory

Thanks in advance.

via Daniel B. Davis