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clear-sidebar-filter-button"><span class="button-wrap"><span class="icon icon-cancel "></span><span class="visually-hidden">Clear filter input</span></span></button></div></section></header><div class="sidebar-inner-nav"><div class="in-nav-toc"><div class="document-toc-container"><section class="document-toc"><header><h2 class="document-toc-heading">In this article</h2></header><ul class="document-toc-list"><li class="document-toc-item "><a class="document-toc-link" href="#modifying_pages_that_match_a_url_pattern">Modifying pages that match a URL pattern</a></li><li class="document-toc-item "><a class="document-toc-link" href="#modifying_pages_programmatically">Modifying pages programmatically</a></li><li class="document-toc-item "><a class="document-toc-link" href="#messaging">Messaging</a></li><li class="document-toc-item "><a class="document-toc-link" href="#learn_more">Learn more</a></li></ul></section></div></div><div class="sidebar-body"> <ol> <li class="section"><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions">Browser extensions</a></li> <li> <details> <summary>Getting started</summary> <ol> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/What_are_WebExtensions">What are extensions?</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Your_first_WebExtension">Your first extension</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Your_second_WebExtension">Your second extension</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Anatomy_of_a_WebExtension">Anatomy of an extension</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Examples">Example extensions</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/What_next">What next?</a></li> </ol> </details> </li> <li> <details> <summary>Concepts</summary> <ol> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API">JavaScript APIs</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Content_scripts">Content scripts</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Background_scripts">Background scripts</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Match_patterns">Match patterns</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Working_with_files">Work with files</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Internationalization">Internationalization</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Content_Security_Policy">Content Security Policy</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Native_messaging">Native messaging</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Differences_between_API_implementations">Differences between API implementations</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Chrome_incompatibilities">Chrome incompatibilities</a></li> </ol> </details> </li> <li> <details> <summary>User interface</summary> <ol> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/user_interface">User interface</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/user_interface/Toolbar_button">Toolbar button</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/user_interface/Page_actions">Address bar button</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/user_interface/Sidebars">Sidebars</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/user_interface/Context_menu_items">Context menu items</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/user_interface/Options_pages">Options page</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/user_interface/Extension_pages">Extension pages</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/user_interface/Notifications">Notifications</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/user_interface/Omnibox">Address bar suggestions</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/user_interface/devtools_panels">devtools panels</a></li> </ol> </details> </li> <li> <details open=""> <summary>How to</summary> <ol> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Intercept_HTTP_requests">Intercept HTTP requests</a></li> <li><em><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Modify_a_web_page" aria-current="page">Modify a web page</a></em></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Safely_inserting_external_content_into_a_page">Insert external content</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Sharing_objects_with_page_scripts">Share objects with page scripts</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Add_a_button_to_the_toolbar">Add a button to the toolbar</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Implement_a_settings_page">Implement a settings page</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Working_with_the_Tabs_API">Work with the Tabs API</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Work_with_the_Bookmarks_API">Work with the Bookmarks API</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Work_with_the_Cookies_API">Work with the Cookies API</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Work_with_contextual_identities">Work with contextual identities</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Interact_with_the_clipboard">Interact with the clipboard</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Extending_the_developer_tools">Extend the developer tools</a></li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Build_a_cross_browser_extension">Build a cross-browser extension</a></li> </ol> </details> </li> <li> <details> <summary>JavaScript APIs</summary> <ol><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Browser_support_for_JavaScript_APIs">Browser support for JavaScript APIs</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/action">action</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/alarms">alarms</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/bookmarks">bookmarks</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/browserAction">browserAction</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/browserSettings">browserSettings</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/browsingData">browsingData</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/captivePortal">captivePortal</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/clipboard">clipboard</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/commands">commands</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/contentScripts">contentScripts</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/contextualIdentities">contextualIdentities</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/cookies">cookies</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/declarativeNetRequest">declarativeNetRequest</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/devtools">devtools</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/dns">dns</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/dom">dom</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/downloads">downloads</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/events">events</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/extension">extension</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/extensionTypes">extensionTypes</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/find">find</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/history">history</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/i18n">i18n</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/identity">identity</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/idle">idle</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/management">management</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/menus">menus</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/notifications">notifications</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/omnibox">omnibox</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/pageAction">pageAction</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/permissions">permissions</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/pkcs11">pkcs11</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/privacy">privacy</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/proxy">proxy</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime">runtime</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/scripting">scripting</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/search">search</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/sessions">sessions</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/sidebarAction">sidebarAction</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/storage">storage</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs">tabs</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/theme">theme</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/topSites">topSites</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/types">types</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/userScripts">userScripts</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/webNavigation">webNavigation</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/webRequest">webRequest</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/windows">windows</a></li></ol> </details> </li> <li> <details> <summary>Manifest keys</summary> <ol> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json">manifest.json</a></li> </ol> <ol><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/action">action</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/author">author</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/background">background</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/browser_action">browser_action</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/browser_specific_settings">browser_specific_settings</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/chrome_settings_overrides">chrome_settings_overrides</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/chrome_url_overrides">chrome_url_overrides</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/commands">commands</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/content_scripts">content_scripts</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/content_security_policy">content_security_policy</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/declarative_net_request">declarative_net_request</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/default_locale">default_locale</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/description">description</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/developer">developer</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/devtools_page">devtools_page</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/dictionaries">dictionaries</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/externally_connectable">externally_connectable</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/homepage_url">homepage_url</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/host_permissions">host_permissions</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/icons">icons</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/incognito">incognito</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/manifest_version">manifest_version</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/name">name</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/offline_enabled">offline_enabled</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/omnibox">omnibox</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/optional_host_permissions">optional_host_permissions</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/optional_permissions">optional_permissions</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/options_page">options_page</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/options_ui">options_ui</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/page_action">page_action</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/permissions">permissions</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/protocol_handlers">protocol_handlers</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/short_name">short_name</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/sidebar_action">sidebar_action</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/storage">storage</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/theme">theme</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/theme_experiment">theme_experiment</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/user_scripts">user_scripts</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/version">version</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/version_name">version_name</a></li><li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/web_accessible_resources">web_accessible_resources</a></li></ol> </details> </li> <li> <details> <summary>Extension Workshop</summary> <ol> <li><a href="https://extensionworkshop.com/documentation/develop/">Develop</a></li> <li><a href="https://extensionworkshop.com/documentation/publish/">Publish</a></li> <li><a href="https://extensionworkshop.com/documentation/manage/">Manage</a></li> <li><a href="https://extensionworkshop.com/documentation/enterprise/">Enterprise</a></li> </ol> </details> </li> <li class="section"><a href="/en-US/docs/Mozilla/Add-ons/Contact_us">Contact us</a></li> <li> <details> <summary>Channels</summary> <ol> <li><a href="https://blog.mozilla.org/addons">Add-ons blog</a></li> <li><a href="https://discourse.mozilla.org/c/add-ons">Add-ons forum</a></li> <li><a href="https://chat.mozilla.org/#/room/%23addons:mozilla.org">Add-ons chat</a></li> </ol> </details> </li> </ol> </div></div><section class="place side"></section></nav></aside><div class="toc-container"><aside class="toc"><nav><div class="document-toc-container"><section class="document-toc"><header><h2 class="document-toc-heading">In this article</h2></header><ul class="document-toc-list"><li class="document-toc-item "><a class="document-toc-link" href="#modifying_pages_that_match_a_url_pattern">Modifying pages that match a URL pattern</a></li><li class="document-toc-item "><a class="document-toc-link" href="#modifying_pages_programmatically">Modifying pages programmatically</a></li><li class="document-toc-item "><a class="document-toc-link" href="#messaging">Messaging</a></li><li class="document-toc-item "><a class="document-toc-link" href="#learn_more">Learn more</a></li></ul></section></div></nav></aside><section class="place side"></section></div></div><main id="content" class="main-content "><article class="main-page-content" lang="en-US"><header><h1>Modify a web page</h1></header><div class="section-content"><p>One of the most common use cases for an extension is to modify a web page. For example, an extension might want to change the style applied to a page, hide particular DOM nodes, or inject extra DOM nodes into the page.</p> <p>There are two ways to do this with WebExtensions APIs:</p> <ul> <li><strong>Declaratively</strong>: Define a pattern that matches a set of URLs, and load a set of scripts into pages whose URL matches that pattern.</li> <li><strong>Programmatically</strong>: Using a JavaScript API, load a script into the page hosted by a particular tab.</li> </ul> <p>Either way, these scripts are called <em>content scripts</em>, and are different from the other scripts that make up an extension:</p> <ul> <li>They only get access to a small subset of the WebExtension APIs.</li> <li>They get direct access to the web page in which they are loaded.</li> <li>They communicate with the rest of the extension using a messaging API.</li> </ul> <p>In this article we'll look at both methods of loading a script.</p></div><section aria-labelledby="modifying_pages_that_match_a_url_pattern"><h2 id="modifying_pages_that_match_a_url_pattern"><a href="#modifying_pages_that_match_a_url_pattern">Modifying pages that match a URL pattern</a></h2><div class="section-content"><p>First of all, create a new directory called "modify-page". In that directory, create a file called "manifest.json", with the following contents:</p> <div class="code-example"><div class="example-header"><span class="language-name">json</span></div><pre class="brush: json notranslate"><code>{ "manifest_version": 2, "name": "modify-page", "version": "1.0", "content_scripts": [ { "matches": ["https://developer.mozilla.org/*"], "js": ["page-eater.js"] } ] } </code></pre></div> <p>The <a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/content_scripts"><code>content_scripts</code></a> key is how you load scripts into pages that match URL patterns. In this case, <code>content_scripts</code> instructs the browser to load a script called "page-eater.js" into all pages under <a href="/">https://developer.mozilla.org/</a>.</p> <div class="notecard note"> <p><strong>Note:</strong> Since the <code>"js"</code> property of <code>content_scripts</code> is an array, you can use it to inject more than one script into matching pages. If you do this the pages share the same scope, just like multiple scripts loaded by a page, and they are loaded in the order that they are listed in the array.</p> </div> <div class="notecard note"> <p><strong>Note:</strong> The <code>content_scripts</code> key also has a <code>"css"</code> property that you can use to inject CSS stylesheets.</p> </div> <p>Next, create a file called "page-eater.js" inside the "modify-page" directory, and give it the following contents:</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>document.body.textContent = ""; let header = document.createElement("h1"); header.textContent = "This page has been eaten"; document.body.appendChild(header); </code></pre></div> <p>Now <a href="https://extensionworkshop.com/documentation/develop/temporary-installation-in-firefox/" class="external" target="_blank">install the extension</a>, and visit <a href="/">https://developer.mozilla.org/</a>. The page should look like this:</p> <p> <img src="/en-US/docs/Mozilla/Add-ons/WebExtensions/Modify_a_web_page/eaten_page.png" alt="developer.mozilla.org page "eaten" by the script" width="1920" height="490" loading="lazy"> </p></div></section><section aria-labelledby="modifying_pages_programmatically"><h2 id="modifying_pages_programmatically"><a href="#modifying_pages_programmatically">Modifying pages programmatically</a></h2><div class="section-content"><p>What if you still want to eat pages, but only when the user asks you to? Let's update this example so we inject the content script when the user clicks a context menu item.</p> <p>First, update "manifest.json" so it has the following contents:</p> <div class="code-example"><div class="example-header"><span class="language-name">json</span></div><pre class="brush: json notranslate"><code>{ "manifest_version": 2, "name": "modify-page", "version": "1.0", "permissions": ["activeTab", "contextMenus"], "background": { "scripts": ["background.js"] } } </code></pre></div> <p>Here, we've removed the <code>content_scripts</code> key, and added two new keys:</p> <ul> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/permissions"><code>permissions</code></a>: To inject scripts into pages we need permissions for the page we're modifying. The <a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/permissions#activetab_permission"><code>activeTab</code> permission</a> is a way to get this temporarily for the currently active tab. We also need the <code>contextMenus</code> permission to be able to add context menu items.</li> <li><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/background"><code>background</code></a>: We're using this to load a persistent <a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Anatomy_of_a_WebExtension#background_scripts">"background script"</a> called <code>background.js</code>, in which we'll set up the context menu and inject the content script.</li> </ul> <p>Let's create this file. Create a new file called <code>background.js</code> in the <code>modify-page</code> directory, and give it the following contents:</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>browser.contextMenus.create({ id: "eat-page", title: "Eat this page", }); browser.contextMenus.onClicked.addListener((info, tab) => { if (info.menuItemId === "eat-page") { browser.tabs.executeScript({ file: "page-eater.js", }); } }); </code></pre></div> <p>In this script we're creating a <a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/menus/create">context menu item</a>, giving it a specific id and title (the text to be displayed in the context menu). Then we set up an event listener so that when the user clicks a context menu item, we check to see if it is our <code>eat-page</code> item. If it is, we inject "page-eater.js" into the current tab using the <a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/executeScript"><code>tabs.executeScript()</code></a> API. This API optionally takes a tab ID as an argument: we've omitted the tab ID, which means that the script is injected into the currently active tab.</p> <p>At this point the extension should look like this:</p> <pre class="brush: plain notranslate">modify-page/ background.js manifest.json page-eater.js </pre> <p>Now <a href="https://extensionworkshop.com/documentation/develop/temporary-installation-in-firefox/#reloading_a_temporary_add-on" class="external" target="_blank">reload the extension</a>, open a page (any page, this time) activate the context menu, and select "Eat this page":</p> <p> <img src="/en-US/docs/Mozilla/Add-ons/WebExtensions/Modify_a_web_page/eat_from_menu.png" alt="Option to eat a page on the context menu" width="1920" height="1186" loading="lazy"> </p></div></section><section aria-labelledby="messaging"><h2 id="messaging"><a href="#messaging">Messaging</a></h2><div class="section-content"><p>Content scripts and background scripts can't directly access each other's state. However, they can communicate by sending messages. One end sets up a message listener, and the other end can then send it a message. The following table summarizes the APIs involved on each side:</p> <figure class="table-container"><table class="fullwidth-table standard-table"> <thead> <tr> <th scope="row"></th> <th scope="col">In content script</th> <th scope="col">In background script</th> </tr> <tr> <th scope="row">Send a message</th> <td><code><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime#sendmessage()">browser.runtime.sendMessage()</a></code></td> <td><code><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/sendMessage">browser.tabs.sendMessage()</a></code></td> </tr> <tr> <th scope="row">Receive a message</th> <td><code><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime/onMessage">browser.runtime.onMessage</a></code></td> <td><code><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime#onmessage">browser.runtime.onMessage</a></code></td> </tr> </thead> </table></figure> <div class="notecard note"> <p><strong>Note:</strong> In addition to this method of communication, which sends one-off messages, you can also use a <a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Content_scripts#connection-based_messaging">connection-based approach to exchange messages</a>. For advice on choosing between the options, see <a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Content_scripts#choosing_between_one-off_messages_and_connection-based_messaging">Choosing between one-off messages and connection-based messaging</a>.</p> </div> <p>Let's update our example to show how to send a message from the background script.</p> <p>First, edit <code>background.js</code> so that it has these contents:</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>browser.contextMenus.create({ id: "eat-page", title: "Eat this page", }); function messageTab(tabs) { browser.tabs.sendMessage(tabs[0].id, { replacement: "Message from the extension!", }); } function onExecuted(result) { let querying = browser.tabs.query({ active: true, currentWindow: true, }); querying.then(messageTab); } browser.contextMenus.onClicked.addListener((info, tab) => { if (info.menuItemId === "eat-page") { let executing = browser.tabs.executeScript({ file: "page-eater.js", }); executing.then(onExecuted); } }); </code></pre></div> <p>Now, after injecting <code>page-eater.js</code>, we use <a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/query"><code>tabs.query()</code></a> to get the currently active tab, and then use <a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/sendMessage"><code>tabs.sendMessage()</code></a> to send a message to the content scripts loaded into that tab. The message has the payload <code>{replacement: "Message from the extension!"}</code>.</p> <p>Next, update <code>page-eater.js</code> like this:</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>function eatPageReceiver(request, sender, sendResponse) { document.body.textContent = ""; let header = document.createElement("h1"); header.textContent = request.replacement; document.body.appendChild(header); } browser.runtime.onMessage.addListener(eatPageReceiver); </code></pre></div> <p>Now, instead of just eating the page right away, the content script listens for a message using <a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime/onMessage"><code>runtime.onMessage</code></a>. When a message arrives, the content script runs essentially the same code as before, except that the replacement text is taken from <code>request.replacement</code>.</p> <p>Since <a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/executeScript"><code>tabs.executeScript()</code></a> is an asynchronous function, and to ensure we send message only after listener has been added in <code>page-eater.js</code>, we use <code>onExecuted()</code> which will be called after <code>page-eater.js</code> executed.</p> <div class="notecard note"> <p><strong>Note:</strong> Press <kbd>Ctrl</kbd>+<kbd>Shift</kbd>+<kbd>J</kbd> (or <kbd>Cmd</kbd>+<kbd>Shift</kbd>+<kbd>J</kbd> on macOS) OR <code>web-ext run --bc</code> to open <a href="https://firefox-source-docs.mozilla.org/devtools-user/browser_console/index.html" class="external" target="_blank">Browser Console</a> to view <code>console.log</code> in background script.</p> <p>Alternatively, use <a class="page-not-created" title="The documentation about this has not yet been written; please consider contributing!" data-href="/en-US/docs/Mozilla/Add-ons/Add-on_Debugger">Add-on Debugger</a> which allows you set breakpoint. There is currently no way to <a href="https://github.com/mozilla/web-ext/issues/759" class="external" target="_blank">start Add-on Debugger directly from web-ext</a>.</p> </div> <p>If we want send messages back from the content script to the background page, we would use <a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime/sendMessage"><code>runtime.sendMessage()</code></a> instead of <a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/sendMessage"><code>tabs.sendMessage()</code></a>, e.g.:</p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>browser.runtime.sendMessage({ title: "from page-eater.js", }); </code></pre></div> <div class="notecard note"> <p><strong>Note:</strong> These examples all inject JavaScript; you can also inject CSS programmatically using the <a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/insertCSS"><code>tabs.insertCSS()</code></a> function.</p> </div></div></section><section aria-labelledby="learn_more"><h2 id="learn_more"><a href="#learn_more">Learn more</a></h2><div class="section-content"><ul> <li> <p><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/Content_scripts">Content scripts</a> guide</p> </li> <li> <p><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/content_scripts"><code>content_scripts</code></a> manifest key</p> </li> <li> <p><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/permissions"><code>permissions</code></a> manifest key</p> </li> <li> <p><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/executeScript"><code>tabs.executeScript()</code></a></p> </li> <li> <p><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/insertCSS"><code>tabs.insertCSS()</code></a></p> </li> <li> <p><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/sendMessage"><code>tabs.sendMessage()</code></a></p> </li> <li> <p><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime/sendMessage"><code>runtime.sendMessage()</code></a></p> </li> <li> <p><a href="/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime/onMessage"><code>runtime.onMessage</code></a></p> </li> <li> <p>Examples using <code>content_scripts</code>:</p> <ul> <li><a href="https://github.com/mdn/webextensions-examples/tree/main/borderify" class="external" target="_blank">borderify</a></li> <li><a href="https://github.com/mdn/webextensions-examples/tree/main/emoji-substitution" class="external" target="_blank">emoji-substitution</a></li> <li><a href="https://github.com/mdn/webextensions-examples/tree/main/notify-link-clicks-i18n" class="external" target="_blank">notify-link-clicks-i18n</a></li> <li><a href="https://github.com/mdn/webextensions-examples/tree/main/page-to-extension-messaging" class="external" target="_blank">page-to-extension-messaging</a></li> </ul> </li> <li> <p>Examples using <code>tabs.executeScript()</code>:</p> <ul> <li><a href="https://github.com/mdn/webextensions-examples/tree/main/beastify" class="external" target="_blank">beastify</a></li> <li><a href="https://github.com/mdn/webextensions-examples/tree/main/context-menu-copy-link-with-types" class="external" target="_blank">context-menu-copy-link-with-types</a></li> </ul> </li> </ul></div></section></article><aside class="article-footer"><div class="article-footer-inner"><div class="svg-container"><svg xmlns="http://www.w3.org/2000/svg" width="162" height="162" viewBox="0 0 162 162" fill="none" role="none"><mask id="b" fill="#fff"><path d="M97.203 47.04c8.113-7.886 18.004-13.871 28.906-17.492a78 78 0 0 1 33.969-3.39c11.443 1.39 22.401 5.295 32.024 11.411s17.656 14.28 23.476 23.86c5.819 9.579 9.269 20.318 10.083 31.385a69.85 69.85 0 0 1-5.387 32.44c-4.358 10.272-11.115 19.443-19.747 26.801-8.632 7.359-18.908 12.709-30.034 15.637l-6.17-21.698c7.666-2.017 14.746-5.703 20.694-10.773 5.948-5.071 10.603-11.389 13.606-18.467a48.14 48.14 0 0 0 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Content available under <a href="/en-US/docs/MDN/Writing_guidelines/Attrib_copyright_license">a Creative Commons license</a>.</p></div></div></footer></div><script type="application/json" id="hydration">{"url":"/en-US/docs/Mozilla/Add-ons/WebExtensions/Modify_a_web_page","doc":{"isMarkdown":true,"isTranslated":false,"isActive":true,"flaws":{},"title":"Modify a web page","mdn_url":"/en-US/docs/Mozilla/Add-ons/WebExtensions/Modify_a_web_page","locale":"en-US","native":"English (US)","sidebarHTML":"\n <ol>\n <li class=\"section\"><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions\">Browser extensions</a></li>\n <li>\n <details>\n <summary>Getting started</summary>\n <ol>\n <li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/What_are_WebExtensions\">What are extensions?</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Your_first_WebExtension\">Your first extension</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Your_second_WebExtension\">Your second extension</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Anatomy_of_a_WebExtension\">Anatomy of an extension</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Examples\">Example extensions</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/What_next\">What next?</a></li>\n </ol>\n </details>\n </li>\n <li>\n <details>\n <summary>Concepts</summary>\n <ol>\n <li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API\">JavaScript APIs</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Content_scripts\">Content scripts</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Background_scripts\">Background scripts</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Match_patterns\">Match patterns</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Working_with_files\">Work with files</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Internationalization\">Internationalization</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Content_Security_Policy\">Content Security Policy</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Native_messaging\">Native messaging</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Differences_between_API_implementations\">Differences between API implementations</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Chrome_incompatibilities\">Chrome incompatibilities</a></li>\n </ol>\n </details>\n </li>\n <li>\n <details>\n <summary>User interface</summary>\n <ol>\n <li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/user_interface\">User interface</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/user_interface/Toolbar_button\">Toolbar button</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/user_interface/Page_actions\">Address bar button</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/user_interface/Sidebars\">Sidebars</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/user_interface/Context_menu_items\">Context menu items</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/user_interface/Options_pages\">Options page</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/user_interface/Extension_pages\">Extension pages</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/user_interface/Notifications\">Notifications</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/user_interface/Omnibox\">Address bar suggestions</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/user_interface/devtools_panels\">devtools panels</a></li>\n </ol>\n </details>\n </li>\n <li>\n <details open=\"\">\n <summary>How to</summary>\n <ol>\n <li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Intercept_HTTP_requests\">Intercept HTTP requests</a></li>\n<li><em><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Modify_a_web_page\" aria-current=\"page\">Modify a web page</a></em></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Safely_inserting_external_content_into_a_page\">Insert external content</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Sharing_objects_with_page_scripts\">Share objects with page scripts</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Add_a_button_to_the_toolbar\">Add a button to the toolbar</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Implement_a_settings_page\">Implement a settings page</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Working_with_the_Tabs_API\">Work with the Tabs API</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Work_with_the_Bookmarks_API\">Work with the Bookmarks API</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Work_with_the_Cookies_API\">Work with the Cookies API</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Work_with_contextual_identities\">Work with contextual identities</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Interact_with_the_clipboard\">Interact with the clipboard</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Extending_the_developer_tools\">Extend the developer tools</a></li>\n<li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Build_a_cross_browser_extension\">Build a cross-browser extension</a></li>\n </ol>\n </details>\n </li>\n <li>\n <details>\n <summary>JavaScript APIs</summary>\n <ol><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Browser_support_for_JavaScript_APIs\">Browser support for JavaScript APIs</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/action\">action</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/alarms\">alarms</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/bookmarks\">bookmarks</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/browserAction\">browserAction</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/browserSettings\">browserSettings</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/browsingData\">browsingData</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/captivePortal\">captivePortal</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/clipboard\">clipboard</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/commands\">commands</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/contentScripts\">contentScripts</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/contextualIdentities\">contextualIdentities</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/cookies\">cookies</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/declarativeNetRequest\">declarativeNetRequest</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/devtools\">devtools</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/dns\">dns</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/dom\">dom</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/downloads\">downloads</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/events\">events</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/extension\">extension</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/extensionTypes\">extensionTypes</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/find\">find</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/history\">history</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/i18n\">i18n</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/identity\">identity</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/idle\">idle</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/management\">management</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/menus\">menus</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/notifications\">notifications</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/omnibox\">omnibox</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/pageAction\">pageAction</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/permissions\">permissions</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/pkcs11\">pkcs11</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/privacy\">privacy</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/proxy\">proxy</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime\">runtime</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/scripting\">scripting</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/search\">search</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/sessions\">sessions</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/sidebarAction\">sidebarAction</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/storage\">storage</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs\">tabs</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/theme\">theme</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/topSites\">topSites</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/types\">types</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/userScripts\">userScripts</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/webNavigation\">webNavigation</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/webRequest\">webRequest</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/windows\">windows</a></li></ol>\n </details>\n </li>\n <li>\n <details>\n <summary>Manifest keys</summary>\n <ol>\n <li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json\">manifest.json</a></li>\n </ol>\n <ol><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/action\">action</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/author\">author</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/background\">background</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/browser_action\">browser_action</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/browser_specific_settings\">browser_specific_settings</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/chrome_settings_overrides\">chrome_settings_overrides</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/chrome_url_overrides\">chrome_url_overrides</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/commands\">commands</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/content_scripts\">content_scripts</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/content_security_policy\">content_security_policy</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/declarative_net_request\">declarative_net_request</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/default_locale\">default_locale</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/description\">description</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/developer\">developer</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/devtools_page\">devtools_page</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/dictionaries\">dictionaries</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/externally_connectable\">externally_connectable</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/homepage_url\">homepage_url</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/host_permissions\">host_permissions</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/icons\">icons</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/incognito\">incognito</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/manifest_version\">manifest_version</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/name\">name</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/offline_enabled\">offline_enabled</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/omnibox\">omnibox</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/optional_host_permissions\">optional_host_permissions</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/optional_permissions\">optional_permissions</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/options_page\">options_page</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/options_ui\">options_ui</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/page_action\">page_action</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/permissions\">permissions</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/protocol_handlers\">protocol_handlers</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/short_name\">short_name</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/sidebar_action\">sidebar_action</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/storage\">storage</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/theme\">theme</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/theme_experiment\">theme_experiment</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/user_scripts\">user_scripts</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/version\">version</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/version_name\">version_name</a></li><li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/web_accessible_resources\">web_accessible_resources</a></li></ol>\n </details>\n </li>\n <li>\n <details>\n <summary>Extension Workshop</summary>\n <ol>\n <li><a href=\"https://extensionworkshop.com/documentation/develop/\">Develop</a></li>\n <li><a href=\"https://extensionworkshop.com/documentation/publish/\">Publish</a></li>\n <li><a href=\"https://extensionworkshop.com/documentation/manage/\">Manage</a></li>\n <li><a href=\"https://extensionworkshop.com/documentation/enterprise/\">Enterprise</a></li>\n </ol>\n </details>\n </li>\n <li class=\"section\"><a href=\"/en-US/docs/Mozilla/Add-ons/Contact_us\">Contact us</a></li>\n <li>\n <details>\n <summary>Channels</summary>\n <ol>\n <li><a href=\"https://blog.mozilla.org/addons\">Add-ons blog</a></li>\n <li><a href=\"https://discourse.mozilla.org/c/add-ons\">Add-ons forum</a></li>\n <li><a href=\"https://chat.mozilla.org/#/room/%23addons:mozilla.org\">Add-ons chat</a></li>\n </ol>\n </details>\n </li>\n </ol>\n","sidebarMacro":"AddonSidebar","body":[{"type":"prose","value":{"id":null,"title":null,"isH3":false,"content":"<p>One of the most common use cases for an extension is to modify a web page. For example, an extension might want to change the style applied to a page, hide particular DOM nodes, or inject extra DOM nodes into the page.</p>\n<p>There are two ways to do this with WebExtensions APIs:</p>\n<ul>\n <li><strong>Declaratively</strong>: Define a pattern that matches a set of URLs, and load a set of scripts into pages whose URL matches that pattern.</li>\n <li><strong>Programmatically</strong>: Using a JavaScript API, load a script into the page hosted by a particular tab.</li>\n</ul>\n<p>Either way, these scripts are called <em>content scripts</em>, and are different from the other scripts that make up an extension:</p>\n<ul>\n <li>They only get access to a small subset of the WebExtension APIs.</li>\n <li>They get direct access to the web page in which they are loaded.</li>\n <li>They communicate with the rest of the extension using a messaging API.</li>\n</ul>\n<p>In this article we'll look at both methods of loading a script.</p>"}},{"type":"prose","value":{"id":"modifying_pages_that_match_a_url_pattern","title":"Modifying pages that match a URL pattern","isH3":false,"content":"<p>First of all, create a new directory called \"modify-page\". In that directory, create a file called \"manifest.json\", with the following contents:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">json</span></div><pre class=\"brush: json notranslate\"><code>{\n \"manifest_version\": 2,\n \"name\": \"modify-page\",\n \"version\": \"1.0\",\n\n \"content_scripts\": [\n {\n \"matches\": [\"https://developer.mozilla.org/*\"],\n \"js\": [\"page-eater.js\"]\n }\n ]\n}\n</code></pre></div>\n<p>The <a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/content_scripts\"><code>content_scripts</code></a> key is how you load scripts into pages that match URL patterns. In this case, <code>content_scripts</code> instructs the browser to load a script called \"page-eater.js\" into all pages under <a href=\"/\">https://developer.mozilla.org/</a>.</p>\n<div class=\"notecard note\">\n <p><strong>Note:</strong> Since the <code>\"js\"</code> property of <code>content_scripts</code> is an array, you can use it to inject more than one script into matching pages. If you do this the pages share the same scope, just like multiple scripts loaded by a page, and they are loaded in the order that they are listed in the array.</p>\n</div>\n<div class=\"notecard note\">\n <p><strong>Note:</strong> The <code>content_scripts</code> key also has a <code>\"css\"</code> property that you can use to inject CSS stylesheets.</p>\n</div>\n<p>Next, create a file called \"page-eater.js\" inside the \"modify-page\" directory, and give it the following contents:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>document.body.textContent = \"\";\n\nlet header = document.createElement(\"h1\");\nheader.textContent = \"This page has been eaten\";\ndocument.body.appendChild(header);\n</code></pre></div>\n<p>Now <a href=\"https://extensionworkshop.com/documentation/develop/temporary-installation-in-firefox/\" class=\"external\" target=\"_blank\">install the extension</a>, and visit <a href=\"/\">https://developer.mozilla.org/</a>. The page should look like this:</p>\n<p>\n <img src=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Modify_a_web_page/eaten_page.png\" alt=\"developer.mozilla.org page "eaten" by the script\" width=\"1920\" height=\"490\" loading=\"lazy\">\n</p>"}},{"type":"prose","value":{"id":"modifying_pages_programmatically","title":"Modifying pages programmatically","isH3":false,"content":"<p>What if you still want to eat pages, but only when the user asks you to? Let's update this example so we inject the content script when the user clicks a context menu item.</p>\n<p>First, update \"manifest.json\" so it has the following contents:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">json</span></div><pre class=\"brush: json notranslate\"><code>{\n \"manifest_version\": 2,\n \"name\": \"modify-page\",\n \"version\": \"1.0\",\n\n \"permissions\": [\"activeTab\", \"contextMenus\"],\n\n \"background\": {\n \"scripts\": [\"background.js\"]\n }\n}\n</code></pre></div>\n<p>Here, we've removed the <code>content_scripts</code> key, and added two new keys:</p>\n<ul>\n <li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/permissions\"><code>permissions</code></a>: To inject scripts into pages we need permissions for the page we're modifying. The <a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/permissions#activetab_permission\"><code>activeTab</code> permission</a> is a way to get this temporarily for the currently active tab. We also need the <code>contextMenus</code> permission to be able to add context menu items.</li>\n <li><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/background\"><code>background</code></a>: We're using this to load a persistent <a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Anatomy_of_a_WebExtension#background_scripts\">\"background script\"</a> called <code>background.js</code>, in which we'll set up the context menu and inject the content script.</li>\n</ul>\n<p>Let's create this file. Create a new file called <code>background.js</code> in the <code>modify-page</code> directory, and give it the following contents:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>browser.contextMenus.create({\n id: \"eat-page\",\n title: \"Eat this page\",\n});\n\nbrowser.contextMenus.onClicked.addListener((info, tab) => {\n if (info.menuItemId === \"eat-page\") {\n browser.tabs.executeScript({\n file: \"page-eater.js\",\n });\n }\n});\n</code></pre></div>\n<p>In this script we're creating a <a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/menus/create\">context menu item</a>, giving it a specific id and title (the text to be displayed in the context menu). Then we set up an event listener so that when the user clicks a context menu item, we check to see if it is our <code>eat-page</code> item. If it is, we inject \"page-eater.js\" into the current tab using the <a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/executeScript\"><code>tabs.executeScript()</code></a> API. This API optionally takes a tab ID as an argument: we've omitted the tab ID, which means that the script is injected into the currently active tab.</p>\n<p>At this point the extension should look like this:</p>\n<pre class=\"brush: plain notranslate\">modify-page/\n background.js\n manifest.json\n page-eater.js\n</pre>\n<p>Now <a href=\"https://extensionworkshop.com/documentation/develop/temporary-installation-in-firefox/#reloading_a_temporary_add-on\" class=\"external\" target=\"_blank\">reload the extension</a>, open a page (any page, this time) activate the context menu, and select \"Eat this page\":</p>\n<p>\n <img src=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Modify_a_web_page/eat_from_menu.png\" alt=\"Option to eat a page on the context menu\" width=\"1920\" height=\"1186\" loading=\"lazy\">\n</p>"}},{"type":"prose","value":{"id":"messaging","title":"Messaging","isH3":false,"content":"<p>Content scripts and background scripts can't directly access each other's state. However, they can communicate by sending messages. One end sets up a message listener, and the other end can then send it a message. The following table summarizes the APIs involved on each side:</p>\n<figure class=\"table-container\"><table class=\"fullwidth-table standard-table\">\n <thead>\n <tr>\n <th scope=\"row\"></th>\n <th scope=\"col\">In content script</th>\n <th scope=\"col\">In background script</th>\n </tr>\n <tr>\n <th scope=\"row\">Send a message</th>\n <td><code><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime#sendmessage()\">browser.runtime.sendMessage()</a></code></td>\n <td><code><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/sendMessage\">browser.tabs.sendMessage()</a></code></td>\n </tr>\n <tr>\n <th scope=\"row\">Receive a message</th>\n <td><code><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime/onMessage\">browser.runtime.onMessage</a></code></td>\n <td><code><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime#onmessage\">browser.runtime.onMessage</a></code></td>\n </tr>\n </thead>\n</table></figure>\n<div class=\"notecard note\">\n <p><strong>Note:</strong> In addition to this method of communication, which sends one-off messages, you can also use a <a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Content_scripts#connection-based_messaging\">connection-based approach to exchange messages</a>. For advice on choosing between the options, see <a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Content_scripts#choosing_between_one-off_messages_and_connection-based_messaging\">Choosing between one-off messages and connection-based messaging</a>.</p>\n</div>\n<p>Let's update our example to show how to send a message from the background script.</p>\n<p>First, edit <code>background.js</code> so that it has these contents:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>browser.contextMenus.create({\n id: \"eat-page\",\n title: \"Eat this page\",\n});\n\nfunction messageTab(tabs) {\n browser.tabs.sendMessage(tabs[0].id, {\n replacement: \"Message from the extension!\",\n });\n}\n\nfunction onExecuted(result) {\n let querying = browser.tabs.query({\n active: true,\n currentWindow: true,\n });\n querying.then(messageTab);\n}\n\nbrowser.contextMenus.onClicked.addListener((info, tab) => {\n if (info.menuItemId === \"eat-page\") {\n let executing = browser.tabs.executeScript({\n file: \"page-eater.js\",\n });\n executing.then(onExecuted);\n }\n});\n</code></pre></div>\n<p>Now, after injecting <code>page-eater.js</code>, we use <a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/query\"><code>tabs.query()</code></a> to get the currently active tab, and then use <a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/sendMessage\"><code>tabs.sendMessage()</code></a> to send a message to the content scripts loaded into that tab. The message has the payload <code>{replacement: \"Message from the extension!\"}</code>.</p>\n<p>Next, update <code>page-eater.js</code> like this:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>function eatPageReceiver(request, sender, sendResponse) {\n document.body.textContent = \"\";\n let header = document.createElement(\"h1\");\n header.textContent = request.replacement;\n document.body.appendChild(header);\n}\nbrowser.runtime.onMessage.addListener(eatPageReceiver);\n</code></pre></div>\n<p>Now, instead of just eating the page right away, the content script listens for a message using <a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime/onMessage\"><code>runtime.onMessage</code></a>. When a message arrives, the content script runs essentially the same code as before, except that the replacement text is taken from <code>request.replacement</code>.</p>\n<p>Since <a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/executeScript\"><code>tabs.executeScript()</code></a> is an asynchronous function, and to ensure we send message only after listener has been added in <code>page-eater.js</code>, we use <code>onExecuted()</code> which will be called after <code>page-eater.js</code> executed.</p>\n<div class=\"notecard note\">\n <p><strong>Note:</strong> Press <kbd>Ctrl</kbd>+<kbd>Shift</kbd>+<kbd>J</kbd> (or <kbd>Cmd</kbd>+<kbd>Shift</kbd>+<kbd>J</kbd> on macOS) OR <code>web-ext run --bc</code> to open <a href=\"https://firefox-source-docs.mozilla.org/devtools-user/browser_console/index.html\" class=\"external\" target=\"_blank\">Browser Console</a> to view <code>console.log</code> in background script.</p>\n <p>Alternatively, use <a class=\"page-not-created\" title=\"The documentation about this has not yet been written; please consider contributing!\" data-href=\"/en-US/docs/Mozilla/Add-ons/Add-on_Debugger\">Add-on Debugger</a> which allows you set breakpoint. There is currently no way to <a href=\"https://github.com/mozilla/web-ext/issues/759\" class=\"external\" target=\"_blank\">start Add-on Debugger directly from web-ext</a>.</p>\n</div>\n<p>If we want send messages back from the content script to the background page, we would use <a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime/sendMessage\"><code>runtime.sendMessage()</code></a> instead of <a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/sendMessage\"><code>tabs.sendMessage()</code></a>, e.g.:</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>browser.runtime.sendMessage({\n title: \"from page-eater.js\",\n});\n</code></pre></div>\n<div class=\"notecard note\">\n <p><strong>Note:</strong> These examples all inject JavaScript; you can also inject CSS programmatically using the <a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/insertCSS\"><code>tabs.insertCSS()</code></a> function.</p>\n</div>"}},{"type":"prose","value":{"id":"learn_more","title":"Learn more","isH3":false,"content":"<ul>\n <li>\n <p><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/Content_scripts\">Content scripts</a> guide</p>\n </li>\n <li>\n <p><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/content_scripts\"><code>content_scripts</code></a> manifest key</p>\n </li>\n <li>\n <p><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/permissions\"><code>permissions</code></a> manifest key</p>\n </li>\n <li>\n <p><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/executeScript\"><code>tabs.executeScript()</code></a></p>\n </li>\n <li>\n <p><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/insertCSS\"><code>tabs.insertCSS()</code></a></p>\n </li>\n <li>\n <p><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/sendMessage\"><code>tabs.sendMessage()</code></a></p>\n </li>\n <li>\n <p><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime/sendMessage\"><code>runtime.sendMessage()</code></a></p>\n </li>\n <li>\n <p><a href=\"/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime/onMessage\"><code>runtime.onMessage</code></a></p>\n </li>\n <li>\n <p>Examples using <code>content_scripts</code>:</p>\n <ul>\n <li><a href=\"https://github.com/mdn/webextensions-examples/tree/main/borderify\" class=\"external\" target=\"_blank\">borderify</a></li>\n <li><a href=\"https://github.com/mdn/webextensions-examples/tree/main/emoji-substitution\" class=\"external\" target=\"_blank\">emoji-substitution</a></li>\n <li><a href=\"https://github.com/mdn/webextensions-examples/tree/main/notify-link-clicks-i18n\" class=\"external\" target=\"_blank\">notify-link-clicks-i18n</a></li>\n <li><a href=\"https://github.com/mdn/webextensions-examples/tree/main/page-to-extension-messaging\" class=\"external\" target=\"_blank\">page-to-extension-messaging</a></li>\n </ul>\n </li>\n <li>\n <p>Examples using <code>tabs.executeScript()</code>:</p>\n <ul>\n <li><a href=\"https://github.com/mdn/webextensions-examples/tree/main/beastify\" class=\"external\" target=\"_blank\">beastify</a></li>\n <li><a href=\"https://github.com/mdn/webextensions-examples/tree/main/context-menu-copy-link-with-types\" class=\"external\" target=\"_blank\">context-menu-copy-link-with-types</a></li>\n </ul>\n </li>\n</ul>"}}],"toc":[{"text":"Modifying pages that match a URL pattern","id":"modifying_pages_that_match_a_url_pattern"},{"text":"Modifying pages programmatically","id":"modifying_pages_programmatically"},{"text":"Messaging","id":"messaging"},{"text":"Learn more","id":"learn_more"}],"summary":"One of the most common use cases for an extension is to modify a web page. For example, an extension might want to change the style applied to a page, hide particular DOM nodes, or inject extra DOM nodes into the page.","popularity":0,"modified":"2024-07-26T03:27:04.000Z","other_translations":[{"locale":"de","title":"Eine Webseite modifizieren","native":"Deutsch"},{"locale":"es","title":"Modify a web page","native":"Español"},{"locale":"fr","title":"Modifier une page web","native":"Français"},{"locale":"ja","title":"ウェブページを変更する","native":"日本語"},{"locale":"ru","title":"Модификация веб страницы","native":"Русский"},{"locale":"zh-CN","title":"修改 web 页面","native":"中文 (简体)"}],"pageType":"guide","source":{"folder":"en-us/mozilla/add-ons/webextensions/modify_a_web_page","github_url":"https://github.com/mdn/content/blob/main/files/en-us/mozilla/add-ons/webextensions/modify_a_web_page/index.md","last_commit_url":"https://github.com/mdn/content/commit/668b38a4f6cd96609b9a969fe4653b46aec4e712","filename":"index.md"},"short_title":"Modify a web page","parents":[{"uri":"/en-US/docs/Mozilla","title":"Mozilla"},{"uri":"/en-US/docs/Mozilla/Add-ons","title":"Add-ons"},{"uri":"/en-US/docs/Mozilla/Add-ons/WebExtensions","title":"Browser extensions"},{"uri":"/en-US/docs/Mozilla/Add-ons/WebExtensions/Modify_a_web_page","title":"Modify a web page"}],"pageTitle":"Modify a web page - Mozilla | MDN","noIndexing":false}}</script></body></html>