How to Inject a Global with Web Extensions in Manifest V3

By  on  

For those of you not familiar with the world of web extension development, a storm is brewing with Chrome. Google will stop support for manifest version 2, which is what the vast majority of web extensions use. Manifest version 3 sees many changes but the largest change is moving from persistent background scripts to service workers. This...is...a...massive...change.

Changes from manifest version 2 to version 3 include:

  • Going from persistent background script to a service worker that can die after 5 minutes
  • No use of <iframe> elements or other DOM APIs from the service worker
  • All APIs have become Promise-based
  • Restrictions on content from a CSP perspective

One function that web extensions often employ is executing scripts upon each new page load. For a web extension like MetaMask, we need to provide a global window.ethereum for dApps to use. So how do we do that with manifest version 3?

As of Chrome v102, developers can define a world property with a value of isolated or main (in the page) for content scripts. While developers should define content_scripts in the extension's manifest.json file, the main value really only works (due to a Chrome bug) when you programmatically define it from the service worker:

await chrome.scripting.registerContentScripts([
  {
    id: 'inpage',
    matches: ['http://*/*', 'https://*/*'],
    js: ['in-page.js'],
    runAt: 'document_start',
    world: 'MAIN',
  },
]);

In the example above, in-page.js is injected and executed within the main content tab every time a new page is loaded. This in-page.js file sets window.ethereum for all dApps to use. If the world is undefined or isolated, the script would still execute but would do so in an isolated environment.

Manifest version 3 work is quite the slog so please hug your closest extension developer. There are many huge structural changes and navigating those changes is a brutal push!

Recent Features

  • By
    Send Text Messages with PHP

    Kids these days, I tell ya.  All they care about is the technology.  The video games.  The bottled water.  Oh, and the texting, always the texting.  Back in my day, all we had was...OK, I had all of these things too.  But I still don't get...

  • By
    An Interview with Eric Meyer

    Your early CSS books were instrumental in pushing my love for front end technologies. What was it about CSS that you fell in love with and drove you to write about it? At first blush, it was the simplicity of it as compared to the table-and-spacer...

Incredible Demos

  • By
    Create a Sheen Logo Effect with CSS

    I was inspired when I first saw Addy Osmani's original ShineTime blog post.  The hover sheen effect is simple but awesome.  When I started my blog redesign, I really wanted to use a sheen effect with my logo.  Using two HTML elements and...

  • By
    jQuery Countdown Plugin

    You've probably been to sites like RapidShare and MegaUpload that allow you to download files but make you wait a specified number of seconds before giving you the download link. I've created a similar script but my script allows you to animate the CSS font-size...

Discussion

    Wrap your code in <pre class="{language}"></pre> tags, link to a GitHub gist, JSFiddle fiddle, or CodePen pen to embed!