Chrome Extension Context Menu: how to append div to page after clicking menu item

Question

Playing around with building a Chrome extension. At the moment I\'ve put together a context menu item. When the context menu item is clicked, it fires itemClicked()

Answer 1

You have probably misunderstood the concept of a background page (and its younger, more resource-friendly and preferred sibling: event page) and that of a content script.

content scripts:

• Are bound to a specific web-page loaded into a tab.
• Live in an isolated world (JS context), but have direct access to the web-pages DOM.
• Can communicate with the background page (see Message Passing).

background pages:

• Are bound to your extension (there is max. 1 background (or event) page for each extension).
• Are always somewhere in the background (event pages "take a nap" from time to time, but you can always wake them up).
• Do not have direct access to any web-page.
• Can communicate with the content scripts (and other views) (see Message Passing).
• Can do cool stuff (because they have access to cool chrome.* APIs).

---

The chrome.contentMenus API is available only to a background page. Thus, you have to create any context menu and listen for onClicked events there (in the background page).
Once a context menu has been clicked, you can use Programmatic Injection to inject some code (or a content script) into the active tab's web-page.

Below is the source code of a sample extension that demonstrates this method.

manifest.json:

{
    "manifest_version": 2,
    "name":    "Test Extension",
    "version": "0.0",

    "background": {
        "persistent": false,   // <-- let's make it an event page
        "scripts": ["background.js"]
    },

    "permissions": [
        "contextMenus",
        "activeTab"   // <-- here, sufficient for our purpose
    ]
}

background.js:

/* Create a context-menu */
chrome.contextMenus.create({
    id: "myContextMenu",   // <-- mandatory with event-pages
    title: "Click me",
    contexts: ["all"]
});

/* Register a listener for the `onClicked` event */
chrome.contextMenus.onClicked.addListener(function(info, tab) {
    if (tab) {
        /* Create the code to be injected */
        var code = [
            'var d = document.createElement("div");',
            'd.setAttribute("style", "'
                + 'background-color: red; '
                + 'width: 100px; '
                + 'height: 100px; '
                + 'position: fixed; '
                + 'top: 70px; '
                + 'left: 30px; '
                + 'z-index: 9999; '
                + '");',
            'document.body.appendChild(d);'
        ].join("\n");

        /* Inject the code into the current tab */
        chrome.tabs.executeScript(tab.id, { code: code });
    }
});

(If your injected code is complicated enough, it might be a better idea to inject a .js file. More info on Programmatic Injection.)