WEB EXTENSION ENGINE

A better way to
build browser
extensions

Epos gives you a zero-config workflow and powerful built-in features that work across all extension contexts with no extra setup.

Install Epos Get Started

WORKFLOW

Install

Install Epos from the Chrome Web Store.

Connect

Connect a local project folder.

Develop

Epos runs your code in the browser.

Export

Click Export to get a ZIP bundle.

Extension Runtime

Epos is not a boilerplate, framework, or bundler. It is a web extension runtime that runs your code directly in the browser during development.

Simple Setup

Just describe which files to load and where.

epos.json

{
  "name": "My Extension",
  "targets": [
    {
      "matches": "*://*.example.com/*",
      "load": ["main.css", "main.js"]
    },
    {
      "matches": "<popup>",
      "load": ["popup.css", "popup.js"]
    },
    {
      "matches": "<background>",
      "load": ["background.js"]
    }
  ]
}

Powerful Features

Cross-context messaging with smart routing
Reactive state with automatic sync
State persistence in IndexedDB
File and data storage
Automatic code injection
Preconfigured Shadow DOM
Simple React integration
CORS-free fetch for web pages
Extension APIs in every context
Invisible web pages for background automation
Granular updates without a full reload
Dedicated project dashboard
One-click export
And plenty more!

Unified Messaging

The simplest API for cross-context communication. Smart automatic routing. Works in every context.

// In the background script:
epos.bus.on('math:sum', (a, b) => a + b)

// On a web page:
const result = await epos.bus.send('math:sum', 1, 2)

State with Real-Time Sync

Update state in one context and see changes instantly in every other context.

const state = await epos.state.connect()

// Modify like a regular object.
// Changes are synced in real time.
state.items = []
state.items.push('Hello world!')

Storage for Files and Data

Store and access files and data from any context. Powered by IndexedDB.

// Store in the background script:
await epos.storage.set('user-pic', imageFile)

// Access later in the popup:
const pic = await epos.storage.get('user-pic')

Works Out of the Box

Every feature works in every extension context. All required setup is handled automatically.

// The same API for every context:
// - background
// - popup
// - side panel
// - web pages
// - iframes
import epos from 'epos'

Start building with Epos

Install Epos and build your next extension.