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weh stands for WebExtensions Helper.

This toolkit speeds up browser add-ons development by providing a number of facilities for WebExtensions-based (Firefox, Chrome, Opera and Edge) extensions.

This is not a framework in the sense that the developer does not have to embrace all the provided utilities and there are not many architectural constraints to follow in order to take benefit of the tool.

The build system generates automatically a directory you can directly install into your browser, compiling automatically CoffeeScript, TypeScript and JSX to Javascript, Sass, Less and Stylus to CSS.

weh also provides some libraries that go into your addon to ease a number of common tasks like managing preferences and two-way communications between the extension background and its user interface content pages, providing a way for the end-user to customize any string in the add-on user interface. Developing the user interface using ReactJS is also simplified but you may choose not to use this library.

In addition, an inspector application (under the form of a weh-based extension) is provided to monitor other weh extensions in real-time.

weh-generated extensions are compatible with Firefox, Chrome, Opera and Edge. You should of course maintain this compatibility in the code you add to your project.

install from npm

npm install -g weh gulp

testing installation

weh init --prjdir myextension

You can now install your skeleton extension from the myextension/build directory as described here.

install from github

npm install -g gulp
git clone
cd weh
npm install
npm link

You can now move away from the weh directory.

using weh

To create a new extension project:

weh init --prjdir myextension

You now have a myextension folder. The myextension/src sub-directory is the place where your add-on specific code goes. After running weh init, the directory contains a simple skeleton code that demonstrates preferences edition. This code is to be modified to do what your extension is supposed to do. The myextension/build contain an add-on ready to be installed into your browser.

To build and maintain the add-on:

cd myextension

You will notice that the last weh command does not return. It is in watch mode, meaning whenever you make a change into the myextension/src directory, those changes are rebuild into myextension/build. If you do not want this behavior and prefer running the build command manually, add --no-watch to the command line.

Run weh help to see more command line options.

installing a local add-on into the browser

  • on Firefox: visit about:debugging, click Load Temporary Addon, select the myextension/build/manifest.json file
  • on Chrome: visit chrome://extension, check Developer mode, click Load unpacked extension, select the myextension/build directory
  • on Opera: visit about:extension, click Developer mode, Load unpacked extension, select myextension/build directory
  • on Edge: (tested with insider Edge version 39.14959) click the 3 dots icon at the top right, select Extensions, click Load extension, select myextension/build directory

extension directory structure

weh expects all project-specific code to be put into the src sub-directory:

  • src/manifest.json: your add-on's manifest
  • src/**/*: those files are processed, so resources like js and css (and other supported languages) are learned and processed to the build directory.
  • src-modules/**/*: files here are used to resolve dependencies
  • locales: files are copied to build/_locales

Also note that you can change the src directory by specifying a directory path with the --srcdir option.

accessing weh services

Declaring weh from a background script: const weh = require('weh-background'); From a content script: const weh = require('weh-content');

You can then access a number of services from the weh variable:

  • weh.rpc: making function calls (both ways) through various components completely transparent: between background and content, background and workers, background and native apps, background and injected-content
  • weh.prefs: preferences system
  • weh.i18n: translation system
  • weh.ui: content management from background utilities

multi-language support

Weh obviously supports Javascript (.js file extension) for scripts and Cascading Style Sheets (.css extension), but you can also use other languages:

  • scripts: JSX (.jsx), Typescript (.ts), Coffee (.coffee)
  • styling: Sass (.scss), Less (.less), Stylus (.styl)

pre-processing files

All files with a .ejs are processed first by an EJS processor. For instance, a file named myscript.js.ejs will be transformed to myscript.js before being processed. You can specify one or several JSON files to provide data for the EJS resolution using the --ejsdata option.

The EJS pre-processing occurs in a first place, so a file named myscript.ts.ejs will first be EJS-processed, then compiled using Typescript, and will end up in the build directory as myscript.js.

Any text file in the src directory can be processed with EJS, not only js and css-like.

Pre-processing is useful if you want to generate different builds from the same source code.

using weh libraries

weh preferences

Preferences are to be formally defined in order to be used in your add-on. An example of preferences description could be:

    name: "myparam_string",
    type: "string",
    defaultValue: "Default value",
    maxLength: 15,
    regexp: "^[a-zA-Z ]+$"
    name: "myparam_integer",
    type: "integer",
    defaultValue: 42,
    minimum: -10,
    maximum: 100
    name: "myparam_float",
    type: "float",
    defaultValue: 3.14159,
    minimum: 1.5,
    maximum: 10.8
    name: "myparam_boolean",
    type: "boolean",
    defaultValue: true
    name: "myparam_choice",
    type: "choice",
    defaultValue: "second",
    choices: [{
        name: "First choice",
        value: "first"
        name: "Second choice",
        value: "second"
        name: "Third choice",
        value: "third"

For each parameter, you must provide at least name, type and defaultValue. type must be one of string, integer, float, boolean or choice. A specific preference parameter can then be accessed, as read or write, through weh.prefs["parameter name"].

You can install preferences listeners using weh.prefs.on(whatToWatch,callback) and un-install listeners using with the same parameters. whatToWatch uses a dotted notation. For instance, listening to "", "a", "a.b" or "a.b.c" will trigger the callback whenever parameter a.b.c is modified. Note that the preferences listeners are available from both background and local content.

You should also define a couple of human viewable strings associated to each parameter in locales/<locale>/messages.json:

  • weh_prefs_label_<parameter name> defines a label for the parameter
  • weh_prefs_description_<parameter name> defines an optional longer description for this parameter

Example (locales/en_US/messages.json):

    "weh_prefs_label_myparam_string": {
        "message": "String parameter"
    "weh_prefs_description_myparam_string": {
        "message": "Only letters and spaces, 20 characters max"

You can define a number of constraints to your preferences. This is useful with the settings user interface provided by weh.

  • maxLength: (type string, integer and float) the number of characters in the input
  • regexp: (type string) a regular expression the string must match
  • minimum: (type integer and float) the minimum acceptable value
  • maximum: (type integer and float) the maximum acceptable value
  • choices: (type choice) the set of possible choices to appear in a select input. This is array of either:
    • object containing fields value (the actual preference value) and name (what is to be displayed to the user)
    • string representing the actual preference value. The label to be displayed for this choice is searched in locales/<locale>/messages.json as weh_prefs_label_<parameter name>_option_<parameter value>

Note that the preferences definition can be declared or updated at any time. This is useful if, for instance, you don't the list of choices in advance.

weh takes care of adding/removing the listener when the component is mounted/unmounted and delivering the message to the onWehMessage method.

debugging tools

The weh toolkit includes an extension called weh-inspector which allows to:

  • monitor messages between the background and UI
  • read/write addon preferences
  • read add-on storage

The weh-inspector is available as a template in the weh toolkit. As such, you can install it with weh init --template inspector --prjdir inspector and then load the generated extension into the browser like any regular weh addon.


weh provides some utilities for dealing with locales.

Instead of browser.i18n.getMessage(), you should use weh._(), with the same parameters:

  • it's shorter
  • it automatically turns character '-' into '_' in string tags while leaving a warning in the console
  • more important: it allows overwriting some or all locale strings. Whenever a call is made to weh._(), the library first searches for a storage-based translation for this tag. If not found, it uses the default string defined in _locales/<locale>/messages.json. By default, weh provides a user interface page for the user to edit locale strings. It is up to the add-on developer to write the code to centralize the user-generated translations on a server, so that it can be shared amongst all users.


weh provides an easy way to call functions across components that do not run within the same threads.

All the functions return promises. If a declared function returns something other than a Promise object, weh takes of promisifying the returned value.

Functions are declared on the called side using weh.rpc.listen() and are called with

For instance, the background can define a function like this:

	my_function: (a,b) => {
		return a + b;

and a content script can call the function this way:"my_function",39,3)

weh.rpc.listen() can declare several functions at once, and can be called several times: only function with the same name are overwritten.

When using the weh.ui module to create a content, for instance creating a tab, a name is given to this content, for instance settings. When the background wants to call a function declared within this content, it must use the content name as the first parameter: `"settings","my_function",39,3);

If the called function does not exists, throw an exception or return explicitly a failed promise the returned promise is rejected.

native messaging

weh is also very useful when dealing with native messaging.

var nativeApp = require('weh-natmsg')("com.example.myapp");"my_function",...params)
		// do something
		// handle error

You can catch all errors due to the native app not being installed (or at least not being callable):

	// for instance, open a tab to a site where to download the app

You can just check whether the app is present, without triggering the onAppNotFound() if it is not:

	// this is called if the app could not be launched

On the native app side, assuming it is developed on node.js, you can use the exact same rpc mechanism, using rpc.listen() and to communicate both ways with the add-on.

For now, the only implementation of such a native is available on the vdhcoapp project under GPL-2.0 license. It is planned to release a version using a less restrictive license.

UI utilities

weh.ui provides the ability to open a tab or a panel, so that the created content can directly be callable from the background using weh.rpc."some_name",{
	url: "content/content.html",
	type: "tab"


Toolkit to save time when developing a browser add-on for Firefox, Chrome, Edge, Opera and Vivaldi







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