Content Scripts

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Content scripts are JavaScript files that run in the context of web pages. By using the standard Document Object Model (DOM), they can read details of the web pages the browser visits, or make changes to them.

Here are some examples of what content scripts can do:

Find unlinked URLs in web pages and convert them into hyperlinks
Increase the font size to make text more legible
Find and process microformat data in the DOM

However, content scripts have some limitations. They cannot:

Use chrome.* APIs (except for parts of chrome.extension)
Use variables or functions defined by their extension's pages
Use variables or functions defined by web pages or by other content scripts
Make cross-site XMLHttpRequests

These limitations aren't as bad as they sound. Content scripts can indirectly use the chrome.* APIs, get access to extension data, and request extension actions by exchanging messages with their parent extension. Content scripts can also communicate with web pages using the shared DOM. For more insight into what content scripts can and can't do, learn about the execution environment.

Manifest

If your content script's code should always be injected, register it in the extension manifest using the content_scripts field, as in the following example.

{
  "name": "My extension",
  ...
  "content_scripts": [
    {
      "matches": ["http://www.google.com/*"],
      "css": ["mystyles.css"],
      "js": ["jquery.js", "myscript.js"]
    }
  ],
  ...
}

If you want to inject the code only sometimes, use the permissions field instead, as described in Programmatic injection.

{
  "name": "My extension",
  ...
  "permissions": [
    "tabs", "http://www.google.com/*"
  ],
  ...
}

Using the content_scripts field, an extension can insert multiple content scripts into a page; each of these content scripts can have multiple JavaScript and CSS files. Each item in the content_scripts array can have the following properties:

Name	Type	Description
`matches`	array of strings	Required. Controls the pages this content script will be injected into. See Match Patterns for more details on the syntax of these strings.
`css`	array of strings	Optional. The list of CSS files to be injected into matching pages. These are injected in the order they appear in this array, before any DOM is constructed or displayed for the page.
`js`	array of strings	Optional. The list of JavaScript files to be injected into matching pages. These are injected in the order they appear in this array.
`run_at`	string	Optional. Controls when the files in `js` are injected. Can be "document_start", "document_end", or "document_idle". Defaults to "document_idle". In the case of "document_start", the files are injected after any files from `css`, but before any other DOM is constructed or any other script is run. In the case of "document_end", the files are injected immediately after the DOM is complete, but before subresources like images and frames have loaded. In the case of "document_idle", the browser chooses a time to inject scripts between "document_end" and immediately after the `window.onload` event fires. The exact moment of injection depends on how complex the document is and how long it is taking to load, and is optimized for page load speed. Note: With "document_idle", content scripts may not necessarily receive the `window.onload` event, because they may run after it has already fired. In most cases, listening for the `onload` event is unnecessary for content scripts running at "document_idle" because they are guaranteed to run after the DOM is complete. If your script definitely needs to run after `window.onload`, you can check if `onload` has already fired by using the `document.readyState` property.
`all_frames`	boolean	Optional. Controls whether the content script runs in all frames of the matching page, or only the top frame. Defaults to `false`, meaning that only the top frame is matched.

Programmatic injection

Inserting code into a page programmatically is useful when your JavaScript or CSS code shouldn't be injected into every single page that matches the pattern — for example, if you want a script to run only when the user clicks a browser action's icon.

To insert code into a page, your extension must have cross-origin permissions for the page. It also must be able to use the chrome.tabs module. You can get both kinds of permission using the manifest file's permissions field.

Once you have permissions set up, you can inject JavaScript into a page by calling executeScript(). To inject CSS, use insertCSS().

The following code (from the make_page_red example) reacts to a user click by inserting JavaScript into the current tab's page and executing the script.

/* in background.html */
chrome.browserAction.onClicked.addListener(function(tab) {
  chrome.tabs.executeScript(null,
                           {code:"document.body.bgColor='red'"});
});

/* in manifest.json */
"permissions": [
  "tabs", "http://*/*"
],

When the browser is displaying an HTTP page and the user clicks this extension's browser action, the extension sets the page's bgcolor property to 'red'. The result, unless the page has CSS that sets the background color, is that the page turns red.

Usually, instead of inserting code directly (as in the previous sample), you put the code in a file. You inject the file's contents like this:

chrome.tabs.executeScript(null, {file: "content_script.js"});

Execution environment

Content scripts execute in a special environment called an isolated world. They have access to the DOM of the page they are injected into, but not to any JavaScript variables or functions created by the page. It looks to each content script as if there is no other JavaScript executing on the page it is running on. The same is true in reverse: JavaScript running on the page cannot call any functions or access any variables defined by content scripts.

For example, consider this simple page:

hello.html
==========
<html>
  <button id="mybutton">click me</button>
  <script>
    var greeting = "hello, ";
    var button = document.getElementById("mybutton");
    button.person_name = "Bob";
    button.addEventListener("click", function() {
      alert(greeting + button.person_name + ".");
    }, false);
  </script>
</html>

Now, suppose this content script was injected into hello.html:

contentscript.js
================
var greeting = "hola, ";
var button = document.getElementById("mybutton");
button.person_name = "Roberto";
button.addEventListener("click", function() {
  alert(greeting + button.person_name + ".");
}, false);

Now, if the button is pressed, you will see both greetings.

Isolated worlds allow each content script to make changes to its JavaScript environment without worrying about conflicting with the page or with other content scripts. For example, a content script could include JQuery v1 and the page could include JQuery v2, and they wouldn't conflict with each other.

Another important benefit of isolated worlds is that they completely separate the JavaScript on the page from the JavaScript in extensions. This allows us to offer extra functionality to content scripts that should not be accessible from web pages without worrying about web pages accessing it.

Communication with the embedding page

Although the execution environments of content scripts and the pages that host them are isolated from each other, they share access to the page's DOM. If the page wishes to communicate with the content script (or with the extension via the content script), it must do so through the shared DOM.

An example can be accomplished using custom DOM events and storing data in a known location. Consider:

http://foo.com/example.html
===========================
var customEvent = document.createEvent('Event');
customEvent.initEvent('myCustomEvent', true, true);

function fireCustomEvent(data) {
  hiddenDiv = document.getElementById('myCustomEventDiv');
  hiddenDiv.innerText = data
  hiddenDiv.dispatchEvent(customEvent);
}

contentscript.js
================
var port = chrome.extension.connect();

document.getElementById('myCustomEventDiv').addEventListener('myCustomEvent', function() {
  var eventData = document.getElementById('myCustomEventDiv').innerText;
  port.postMessage({message: "myCustomEvent", values: eventData});
});

In the above example, example.html (which is not a part of the extension) creates a custom event and then can decide to fire the event by setting the event data to a known location in the DOM and then dispatching the custom event. The content script listens for the name of the custom event on the known element and handles the event by inspecting the data of the element, and turning around to post the message to the extension process. In this way the page establishes a line of communication to the extension. The reverse is possible through similar means.

Security considerations

When writing a content script, you should be aware of two security issues. First, be careful not to introduce security vulnerabilities into the web site your content script is injected into. For example, if your content script receives content from another web site (e.g., by asking your background page to make an XMLHttpRequest), be careful to filter that content for cross-site scripting attacks before injecting the content into the current page. For example, prefer to inject content via innerText rather than innerHTML. Be especially careful when retrieving HTTP content on an HTTPS page because the HTTP content might have been corrupted by a network "man-in-the-middle" if the user is on a hostile network.

Second, although running your content script in an isolated world provides some protection from the web page, a malicious web page might still be able to attack your content script if you use content from the web page indiscriminately. For example, the following patterns are dangerous:

contentscript.js
================
var data = document.getElementById("json-data")
// WARNING! Might be evaluating an evil script!
var parsed = eval("(" + data + ")")

contentscript.js
================
var elmt_id = ...
// WARNING! elmt_id might be "); ... evil script ... //"!
window.setTimeout("animate(" + elmt_id + ")", 200);

Instead, prefer safer APIs that do not run scripts:

contentscript.js
================
var data = document.getElementById("json-data")
// JSON.parse does not evaluate the attacker's scripts.
var parsed = JSON.parse(data)

contentscript.js
================
var elmt_id = ...
// The closure form of setTimeout does not evaluate scripts.
window.setTimeout(function() {
  animate(elmt_id);
}, 200);

Referring to extension files

Get the URL of an extension's file using chrome.extension.getURL(). You can use the result just like you would any other URL, as the following code shows.

//Code for displaying <extensionDir>/images/myimage.png:
var imgURL = chrome.extension.getURL("images/myimage.png");
document.getElementById("someImage").src = imgURL;

Examples

The contentscript_xhr example shows how an extension can perform cross-site requests for its content script. You can find other simple examples of communication via messages in the examples/api/messaging directory.

See make_page_red and email_this_page for examples of programmatic injection.

For more examples and for help in viewing the source code, see Samples.

Videos

The following videos discuss concepts that are important for content scripts. The first video describes content scripts and isolated worlds.

The next video describes message passing, featuring an example of a content script sending a request to its parent extension.