Error: Cannot find module 'express'

If you encounter the following error:

$ node server.js

module.js:340
    throw err;
          ^
Error: Cannot find module 'express'
    at Function.Module._resolveFilename (module.js:338:15)
    at Function.Module._load (module.js:280:25)
    at Module.require (module.js:364:17)
    at require (module.js:380:17)
    at Object.<anonymous> (/Users/izeye/IdeaProjects/samples-reactjs/samples/docs/tutorial/server.js:3:15)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Function.Module.runMain (module.js:497:10)
$

install `express` manually as follows:

npm install express

or use `package.json`'s `dependencies` as follows:

{
  "name": "tutorial",
  "version": "1.0.0",
  "description": "",
  "main": "server.js",
  "dependencies": {
    "body-parser": "^1.4.3",
    "express": "^4.4.5"
  },
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1",
    "start": "node server.js"
  },
  "author": "",
  "license": "ISC"
}

and run `npm install`.