Access variables and functions defined in page context using a content script

I’m learning how to create Chrome extensions. I just started developing one to catch YouTube events. I want to use it with YouTube flash player (later I will try to make it compatible with HTML5).

manifest.json:

{
    "name": "MyExtension",
    "version": "1.0",
    "description": "Gotta catch Youtube events!",
    "permissions": ["tabs", "http://*/*"],
    "content_scripts" : [{
        "matches" : [ "www.youtube.com/*"],
        "js" : ["myScript.js"]
    }]
}

myScript.js:

function state() { console.log("State Changed!"); }
var player = document.getElementById("movie_player");
player.addEventListener("onStateChange", "state");
console.log("Started!");

The problem is that the console gives me the “Started!”, but there is no “State Changed!” when I play/pause YouTube videos.

When this code is put in the console, it worked. What am I doing wrong?

Answer

Underlying cause:
Content scripts are executed in an “isolated world” environment.

Solution:
Inject the code into the page using DOM – that code will be able to access functions/variables of the page context (“main world”) or expose functions/variables to the page context (in your case it’s the state() method).

• Note in case communication with the page script is needed:
  Use DOM CustomEvent handler. Examples: one, two, and three.

• Note in case chrome API is needed in the page script:
  Since chrome.* APIs can’t be used in the page script, you have to use them in the content script and send the results to the page script via DOM messaging (see the note above).

Safety warning:
A page may redefine or augment/hook a built-in prototype so your exposed code may fail if the page did it in an incompatible fashion. If you want to make sure your exposed code runs in a safe environment then you should either a) declare your content script with “run_at”: “document_start” and use Methods 2-3 not 1, or b) extract the original native built-ins via an empty iframe, example. Note that with document_start you may need to use DOMContentLoaded event inside the exposed code to wait for DOM.

Table of contents

• Method 1: Inject another file – ManifestV3 compatible
• Method 2: Inject embedded code – MV2
• Method 2b: Using a function – MV2
• Method 3: Using an inline event – ManifestV3 compatible
• Method 4: Using executeScript’s world – ManifestV3 only
• Method 5: Using world in manifest.json – ManifestV3 only, Chrome 111+
• Dynamic values in the injected code

Method 1: Inject another file (ManifestV3/MV2)

Particularly good when you have lots of code. Put the code in a file within your extension, say script.js. Then load it in your content script like this:

var s = document.createElement('script');
s.src = chrome.runtime.getURL('script.js');
s.onload = function() {
    this.remove();
};
(document.head || document.documentElement).appendChild(s);

The js file must be exposed in web_accessible_resources:

• manifest.json example for ManifestV2

  "web_accessible_resources": ["script.js"],
  

• manifest.json example for ManifestV3

  "web_accessible_resources": [{
    "resources": ["script.js"],
    "matches": ["<all_urls>"]
  }]
  

If not, the following error will appear in the console:

Denying load of chrome-extension://[EXTENSIONID]/script.js. Resources must be listed in the web_accessible_resources manifest key in order to be loaded by pages outside the extension.

Method 2: Inject embedded code (MV2)

This method is useful when you want to quickly run a small piece of code. (See also: How to disable facebook hotkeys with Chrome extension?).

var actualCode = `// Code here.
// If you want to use a variable, use $ and curly braces.
// For example, to use a fixed random number:
var someFixedRandomValue = ${ Math.random() };
// NOTE: Do not insert unsafe variables in this way, see below
// at "Dynamic values in the injected code"
`;

var script = document.createElement('script');
script.textContent = actualCode;
(document.head||document.documentElement).appendChild(script);
script.remove();

Note: template literals are only supported in Chrome 41 and above. If you want the extension to work in Chrome 40-, use:

var actualCode = ['/* Code here. Example: */' + 'alert(0);',
                  '// Beware! This array have to be joined',
                  '// using a newline. Otherwise, missing semicolons',
                  '// or single-line comments (//) will mess up your',
                  '// code ----->'].join('n');

Method 2b: Using a function (MV2)

For a big chunk of code, quoting the string is not feasible. Instead of using an array, a function can be used, and stringified:

var actualCode = '(' + function() {
    // All code is executed in a local scope.
    // For example, the following does NOT overwrite the global `alert` method
    var alert = null;
    // To overwrite a global variable, prefix `window`:
    window.alert = null;
} + ')();';
var script = document.createElement('script');
script.textContent = actualCode;
(document.head||document.documentElement).appendChild(script);
script.remove();

This method works, because the + operator on strings and a function converts all objects to a string. If you intend on using the code more than once, it’s wise to create a function to avoid code repetition. An implementation might look like:

function injectScript(func) {
    var actualCode = '(' + func + ')();'
    ...
}
injectScript(function() {
   alert("Injected script");
});

Note: Since the function is serialized, the original scope, and all bound properties are lost!

var scriptToInject = function() {
    console.log(typeof scriptToInject);
};
injectScript(scriptToInject);
// Console output:  "undefined"

Method 3: Using an inline event (ManifestV3/MV2)

Sometimes, you want to run some code immediately, e.g. to run some code before the <head> element is created. This can be done by inserting a <script> tag with textContent (see method 2/2b).

An alternative, but not recommended is to use inline events. It is not recommended because if the page defines a Content Security policy that forbids inline scripts, then inline event listeners are blocked. Inline scripts injected by the extension, on the other hand, still run. If you still want to use inline events, this is how:

var actualCode = '// Some code example n' + 
                 'console.log(document.documentElement.outerHTML);';

document.documentElement.setAttribute('onreset', actualCode);
document.documentElement.dispatchEvent(new CustomEvent('reset'));
document.documentElement.removeAttribute('onreset');

Note: This method assumes that there are no other global event listeners that handle the reset event. If there is, you can also pick one of the other global events. Just open the JavaScript console (F12), type document.documentElement.on, and pick on of the available events.

Method 4: Using chrome.scripting API world (ManifestV3 only)

• Chrome 95 or newer, chrome.scripting.executeScript with world: 'MAIN'
• Chrome 102 or newer, chrome.scripting.registerContentScripts with world: 'MAIN', also allows runAt: 'document_start' to guarantee early execution of the page script.

Unlike the other methods, this one is for the background script or the popup script, not for the content script. See the documentation and examples.

Method 5: Using world in manifest.json (ManifestV3 only)

In Chrome 111 or newer you can add "world": "MAIN" to content_scripts declaration in manifest.json to override the default value which is ISOLATED. The scripts run in the listed order.

  "content_scripts": [{
    "js": ["content.js"],
    "matches": ["<all_urls>"],
    "run_at": "document_start"
  }, {
    "world": "MAIN",
    "js": ["page.js"],
    "matches": ["<all_urls>"],
    "run_at": "document_start"
  }],

Dynamic values in the injected code (MV2)

Occasionally, you need to pass an arbitrary variable to the injected function. For example:

var GREETING = "Hi, I'm ";
var NAME = "Rob";
var scriptToInject = function() {
    alert(GREETING + NAME);
};

To inject this code, you need to pass the variables as arguments to the anonymous function. Be sure to implement it correctly! The following will not work:

var scriptToInject = function (GREETING, NAME) { ... };
var actualCode = '(' + scriptToInject + ')(' + GREETING + ',' + NAME + ')';
// The previous will work for numbers and booleans, but not strings.
// To see why, have a look at the resulting string:
var actualCode = "(function(GREETING, NAME) {...})(Hi, I'm ,Rob)";
//                                                 ^^^^^^^^ ^^^ No string literals!

The solution is to use JSON.stringify before passing the argument. Example:

var actualCode = '(' + function(greeting, name) { ...
} + ')(' + JSON.stringify(GREETING) + ',' + JSON.stringify(NAME) + ')';

If you have many variables, it’s worthwhile to use JSON.stringify once, to improve readability, as follows:

...
} + ')(' + JSON.stringify([arg1, arg2, arg3, arg4]).slice(1, -1) + ')';

Dynamic values in the injected code (ManifestV3)

• Method 1 can set the URL of the script element in the content script:

  s.src = chrome.runtime.getURL('script.js?') + new URLSearchParams({foo: 1});
  

  Then script.js can read it:

  const params = new URLSearchParams(document.currentScript.src.split('?')[1]);
  console.log(params.get('foo'));
  

• Method 4 executeScript has args parameter, registerContentScripts currently doesn’t (hopefully it’ll be added in the future).