Chrome Extension Socket io node js

Since you only need the socket.io-client, this is what you should be doing:

"background": {
  "scripts": [
    "socket.io.js",
    "background.js"
  ]
},

Download and add the socket.io.js file from here: https://raw.githubusercontent.com/Automattic/socket.io-client/1.3.5/socket.io.js

Note in 2020:

Rahat's answer is fine except the socket version , when you use it in background script it always reconnects , so if you use inappropriate version of socket.io.js you 'll get something like this

but with socket.io.js 2.3.0 worked correctly as it should here is a link which worked for me https://cdn.jsdelivr.net/npm/socket.io-client@2/dist/socket.io.js

More info at https://www.npmjs.com/package/socket.io-client