I have been learning Svelte. I find it extremely easy to work with. It is lightweight and fast, a suitable option for building small-scaled apps and websites. I've also been learning about how a browser extension is made. Then I find it a perfect playmate for Svelte, and the learning paths crossed. So today I would like to share my process of building a complete cross-browser extension with Svelte.

Before reading on, make sure that you're familiar with a browser extension and how to construct one with pure HTML/CSS/JS, and how to work with a front-end library/framework in a NodeJS environment. Otherwise, here are some resources to get you started:

• Chrome Extension - Getting started (Manifest v2)
• Svelte - Getting started
• Build a cross-browser extension
• Extensionizr - Chrome extension structure generator

1. Setup

1.1. Initiate a Svelte project

From the CLI, run npx degit sveltejs/template my-extension. This will make a copy of the Svelte template to your machine with the name my-extension, and a basic structure as seen in the repository.

1.2. Install dependencies

Run npm install to install all necessary dependencies in the template's package.json.

To build a cross-browser extension, the webextension-polyfill is required. Therefore, run npm install --save-dev webextension-polyfill to install it.

2. Prepare the Manifest

Now that we have things ready. Let's create the heart of a browser extension, the manifest.json.

Create manifest.json and place it inside the public folder. A bare minimum content of a manifest is:

{
  "manifest_version": 2,
  "name": "My Extension",
  "version": "1.0.0",
}

And depending on the purpose and functionality of the extension, other fields may be specified. For a complete list of supported fields, refer to Chrome's manifest file format.

3. Add functionalities

Silly things that our extension will be capable of:

• Say hello when somebody installs our extension (background page)
• "Hack" the website's background (content script)
• Configure the desired background (popup page)

Now let's dig through.

3.1. Background page

The background page will be loaded whenever the extension is active, and react to the events we set. In our case, the extension reacts "say hello" to the event "somebody installs the extension".

Create background.js inside the src folder and add the following script:

import browser from "webextension-polyfill";

browser.runtime.onInstalled.addListener(({ reason }) => {
  if (reason === "install") {
    alert("Hello");
  }
});

More about the runtime.onInstalled event: MDN.

3.2. Content script

The content script has direct access to the current web page. Therefore, it is a perfect solution to "hack the website's background".

Create injection.js inside the src folder and add the following script:

import browser from "webextension-polyfill";

const key = "background";
browser.storage.local.get(key).then((data) => {
  document.body.style = `background: url(${data[key]})`;
});

More about the storage.local API: MDN.

3.3. Popup page

The popup page is literally the front end of the extension, where users can interact with our extension. In our case, we give users the possibility to "configure the desired background". And this is where Svelte comes into the play.

Remove props: { name: 'world' } from main.js.

Replace content in App.svelte with the following script:

<script>
  import browser from "webextension-polyfill";

  let image = "https://images.unsplash.com/photo-1586074299757-dc655f18518c?fit=crop&w=1268&q=80";

  function change() {
    browser.storage.local.set({ background: image });
  }
</script>

<main>
  Image URL: <input type="text" bind:value={image} />
  <button on:click={change}>Change</button>
</main>

3.4. Add to the Manifest

Now modify the manifest.json we created earlier to include these functionalities:

{
  ...
  "permissions": ["storage"],
  "background": {
    "scripts": ["build/background.js"],
    "persistent": false
  },
  "browser_action": {
    "default_popup": "index.html"
  },
  "content_scripts": [
    {
      "matches": ["https://*/*"],
      "js": ["build/injection.js"]
    }
  ]
}

4. Config Rollup and build

The final step involves some configuration for Rollup to generate the background.js and injection.js to the build folder, so that the Manifest could recognize.

By default, the rollup.config.js file only outputs the main.js entry to the build folder under the alias bundle.js. To include the other scripts, modify the config as follows:

export default [
  {
    input: "src/main.js",
    ...
  },
  {
    input: "src/background.js",
    output: {
      sourcemap: true,
      format: "iife",
      file: "public/build/background.js",
    },
    plugins: [resolve(), commonjs()],
    watch: {
      clearScreen: false,
    },
  },
  {
    input: "src/injection.js",
    output: {
      sourcemap: true,
      format: "iife",
      file: "public/build/injection.js",
    },
    plugins: [resolve(), commonjs()],
    watch: {
      clearScreen: false,
    },
  },
]

This will output multiple entries, including the background.js and injection.js to the build folder, and resolve any dependencies in our scripts.

And finally, run npm run build to compile our project and everything will be available in the public folder. We may then load our extension in Chrome and see the result:

A complete repo you can find at: https://github.com/khang-nd/browser-extension-svelte

Thank you for reading, and see you in the next post.