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
    Regular Expressions for the Rest of Us

    Sooner or later you'll run across a regular expression. With their cryptic syntax, confusing documentation and massive learning curve, most developers settle for copying and pasting them from StackOverflow and hoping they work. But what if you could decode regular expressions and harness their power? In...

  • By
    Create a CSS Cube

    CSS cubes really showcase what CSS has become over the years, evolving from simple color and dimension directives to a language capable of creating deep, creative visuals.  Add animation and you've got something really neat.  Unfortunately each CSS cube tutorial I've read is a bit...

Incredible Demos

  • By
    Animated AJAX Record Deletion Using jQuery

    I'm a huge fan of WordPress' method of individual article deletion. You click the delete link, the menu item animates red, and the item disappears. Here's how to achieve that functionality with jQuery JavaScript. The PHP - Content & Header The following snippet goes at the...

  • By
    MooTools PulseFade Plugin

    I was recently driven to create a MooTools plugin that would take an element and fade it to a min from a max for a given number of times. Here's the result of my Moo-foolery. The MooTools JavaScript Options of the class include: min: (defaults to .5) the...

Discussion

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