Create Firefox Extension Using KotlinJS

Create Firefox Extension Using KotlinJS

Kotlin, my favourite programming language, has seen a fast adoption on platforms like Android or Server where the code is compiled to Java bytecode. However, as Kotlin also supports compilation to Javascript, the language is starting to receive attention in the Web ecosystem as well. So today i am going to show you How to create Firefox Extension Using KotlinJS.

While some content on writing web applications in Kotlin has been published, the niche of writing browser extensions has seemed to be ignored until now. That’s why in this post we are going explore the process of writing a simple extension for Firefox using Kotlin JS.

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How to Create Firefox Extension Using KotlinJS?

The extension is based on Your first extension by Mozilla and will simply add a red border to the website.


For testing our Firefox extension, we will use the web-ext tool from Mozilla. To obtain it you need to install node.js and then run the following command in a terminal:

npm install --global web-ext

Apart from that, we will be using IntelliJ IDEA 2017.2.6 with the Kotlin plugin version 1.1.60 to develop the extension.

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2Setting up the project with KotlinJS

Creating a new project using the KotlinJS (JavaScript) variant

In Intelli IDEA, we create a new Gradle based project and check the Kotlin (JavaScript) option.

We proceed through the wizard until the empty project is created and opened.

As the next step, we will enable the Dead Code Elimination plugin for KotlinJS. This is necessary because a Kotlin JS program needs to bundle the Kotlin stdlib which is over a megabyte in size. However, we can reduce the size of our compiled code drastically by removing code that’s never getting called.

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To enable the plugin, simply add the following line to the build.gradle file.

apply plugin: 'kotlin-dce-js'

3Creating the extension manifest

In the next step, we add the manifest file which is required for a Firefox extension. This file is called manifest.json and resided in the root directory of our project. The file should have the following content.

  "manifest_version": 2,
  "name": "Kotlin Borderify",
  "version": "1.0",
  "description": "Adds a red border to",
  "content_scripts": [
      "matches": [
      "js": [

In this file we declare that our extension will inject a content script in any website matching the pattern *://* . The necessary script files are are the Kotlin stdlib kotlin.js as well as our code in the kt-borderify.js file.

4The code

Next, we create a file main.kt in the src/main/kotlin directory. This code will be run, whenever a matching site will be loaded. The entry point for our code is a standard main function. Inside it, we access the body of the document and apply a border style.

import kotlin.browser.document

fun main(args: Array<String>) {
    document.body?.style?.border = "5px solid red"

The code inside the main function is similar to the corresponding Javascript code. However, we see that because the body of a document can be absent, the type system is forcing us to use the safe-call operator ?. to prevent an exception. This is a glimpse of the language features that Kotlin provides to make developing in it eaiser and the resulting code safer.

5Testing the extension

Intellij IDEA run configuration for continuously compiling the Kotlin JS code

Now it’s time to test our extension. First, we need to compile our code and shrink it by eliminating dead code. This is done by running the Gradle task runDceKotlinJs . We run the task in continous mode so that when we make addtional changes to the code, it gets recompiled immediately.

You can run the Gradle task from IntelliJ IDEA by creating a run configuration like so

or from the command line by running

./gradlew runDceKotlinJs --continuous

Next, we use the web-ext tool start a new Firefox instance with our extension installed by running

web-ext run

from a terminal. Hint: IntelliJ IDEA has a built-in terminal.

In the just launched browser, we navigate to We are greeted with a beautiful red border to demonstrate that the extension is working.


Now, let’s change the color of the border from red to green. To do so, we modify our code to

document.body?.style?.border = "5px solid green"

When we switch back to Firefox, we see (after a short delay) that our changes are automatically applied without us having to run any commands.

6Download Source code

Download from GitHub


In this post we saw how to write a simple Firefox extension in Kotlin JS that injects a content script. The setup was fairly easy and we didn’t hit any major roadblocks. Additionally, the workflow including continuous building and live reloading the extension turned out to be pretty comfortable.

As a next step, it would be interesting to build an extension that interfaces with the Web Extensions API, e.g. by creating a toolbar button.

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