Help with raspberry pi slack camera

Hi everyone,
I’m trying to follow this guide:
and am having problem trying to get this command (node index.js) to run successfully.I’m getting this error:

Error: Cannot find module 'express'
    at Function.Module._resolveFilename (module.js:555:15)
    at Function.Module._load (module.js:482:25)
    at Module.require (module.js:604:17)
    at require (internal/module.js:11:18)
    at Object.<anonymous> (/home/pi/slackcam/slackcam/index.js:1:82)
    at Module._compile (module.js:660:30)
    at Object.Module._extensions..js (module.js:671:10)
    at Module.load (module.js:573:32)
    at tryModuleLoad (module.js:513:12)
    at Function.Module._load (module.js:505:3)

I’m new to java script and iot connected devices so If anyone could help that would be greatly appreciated.

You are missing the module express.js, and need to install it with npm. Refer to the express website.