Using AngularJS, Google Chrome extensions, and the Shadow DOM

I have been working on a Google Chrome extension that will display either the web page that the user is viewing, or replace it with something else, given a certain condition. Rather than muck around with overlays, or wrapping the page content in a div to be hidden and revealed, I elected for something much simpler, and seemingly suited perfectly to the job at hand: the Shadow DOM.

Shadow DOM

The Shadow DOM specification is (as of February 2015) a work in process, but has been implemented in some browsers, including Google Chrome.

The Shadow DOM allows you to take a host element, say a particular div, the body, or even the document element itself, and create a shadow root.

Creating a shadow root

var shadowRoot = document.documentElement.createShadowRoot();

Executing the expression above will cause the (compliant) browser to immediately render the contents of the newly created shadow root. Since this newly created shadow root is empty, that is exactly what the browser renders.

Projecting content from the host

You can select content to project from the host element on to the shadow root with the content tag and select attribute.

<content select='*'></content>

Injecting the fragment above in to the shadow root will cause the browser to project the entire contents of the host (the documentElement) on to the shadow root. It is not difficult to imagine how we might use this to switch back and forth between rendering the original web page and something else.

Using Angular to switch between original and something else

No need to rely on our imaginations, however.

Let us say we are browsing on a website with the following HTML:

<!DOCTYPE html>
    <p>Hello, world!</p>

Our Google Chrome extension has the following content script:

var container, module, shadowRoot, viewURL;

container = document.createElement('span');
module = angular.module('myApp', []);
shadowRoot = document.documentElement.createShadowRoot();
viewURL = chrome.runtime.getURL('view.html');

module.controller('MyCtrl', ['$scope', function ($scope) {
    $scope.state = 'off';
    $scope.$on('switch', function () {
        $scope.state = $scope.state === 'off' ? 'on' : 'off';
}).run(['$rootElement', '$rootScope', function ($rootElement, $rootScope) {
    $rootElement.html('<ng-include src="\'' + viewURL + ''\'" />');
    chrome.runtime.onMessage(message, sender, sendResponse) {
        if ('switch' === message) {

angular.bootstrap(container, [module]);

The Google Chrome extension has the following view for the shadow root:

<span ng-controller="MyCtrl">
    <span ng-switch="switch">
        <span ng-switch-when="on">
            <p>Goodbye, world!</p>
        <span ng-switch-when="off">
            <content select="*"></content>



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