Jono Shields

Making a Chrome Extension

First Steps

I had this idea when I was making a sidebar for my blog, it was like if a waterfall was rainbows, wouldn’t it be cool if every website was this joyous?

So I started off making a Bookmarklet (a browser bookmark that contains javascript). The idea is pretty simple and just adds a pretty animated canvas to the DOM.


  var body=document.getElementsByTagName("body");
  var canvas=document.createElement("canvas");
  // Note : This needs to be url encoded to be run as a bookmarklet.
  // Full source : 

Just drag and drop this great emoji link into your bookmark bar or just click it to see its effects.

So that worked really nicely and I was really happy with it, but I want rainbows on every page and I don’t want to have to click a bookmark every time.

Wrapping it in a Chrome Extenion

Enter Chrome Extensions.

Looks easy right? That is what I thought too!

And for something simple like this it was pretty easy. But when I came to add a toggle button to my extension all I found was pain and hardship.

First off you have to make a manifest.json to declare information about your app. This has things like icons, permissions and the scripts that will be executed.


  "name": "Rainbows For All",
  "version": "1.0",
  "description": "Rainbowify your browsing experience!",
  "permissions": ["<all_urls>"],
  "content_scripts": [{
      "js": ["addContent.js"],
      "matches": ["<all_urls>"]
  "icons": { 
    "16": "icon-16.png",
    "32": "icon-32.png",
    "48": "icon-48.png",
    "128": "icon-128.png" },
  "web_accessible_resources": ["rainbow.js"],
    "browser_action": {
   "default_icon": {
       "32": "icon-32.png"
   "default_title": "Rainbows!"
  "manifest_version": 2

Above you will notice two scripts, addContent.js and rainbow.js. The content_scripts run as part of the chrome extension itself, whereas web_accessible_resources are files that your chrome extension can access.

rainbow.js is just my bookmarklet script from above. addContent.js is the chrome extension itself, all it does is add my rainbow script to the DOM in <script> tags.


var rainbowInstallService = document.createElement('script');
rainbowInstallService.src = chrome.extension.getURL('rainbow.js');
rainbowInstallService.onload = function() {
    if(rainbowInstallService.parentNode != null){

So aside from all the images my extension is just these three files (manifest.json, addContent.js and rainbow.js).

It would have been cool to have built it in a way that allowed the user to turn it on and off. But the Chrome API is quite confusing (read : it was late at night). Also not that many stack overflow posts :(

Source code is here.

Thanks for visiting!