Use a content script to access the page context variables and functions

Question

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 1

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

Solution::
To access functions/variables of the page context ("main world") you have to inject the code that wants to access them into the page itself. Same thing if you want to expose your functions/variables to the page context (in your case it's the state() method).

Note in case chrome API is needed:
Since chrome.* APIs can't be used in the exposed page script directly, you have to use them in the content script and communicate to the exposed page script via a special DOM CustomEvent handler, example one and two. If you don't have to use chrome.* APIs, simply inject all of your JS code in the page by adding a <script> tag as shown below.

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
• Method 2: Inject embedded code
• Method 2b: Using a function
• Method 3: Using an inline event
• Dynamic values in the injected code

Method 1: Inject another file

This is the easiest/best method when you have lots of code. Include your actual JS code in a file within your extension, say script.js. Then let your content script be as follows (explained here: Google Chome “Application Shortcut” Custom Javascript):

var s = document.createElement('script');
// TODO: add "script.js" to web_accessible_resources in manifest.json
s.src = chrome.runtime.getURL('script.js');
s.onload = function() {
    this.remove();
};
(document.head || document.documentElement).appendChild(s);

Note: For security reasons, Chrome prevents loading of js files. Your file must be added as a "web_accessible_resources" item (example) :

// manifest.json must include: 
"web_accessible_resources": ["script.js"],

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

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

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

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.

Dynamic values in the injected code

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]) + ')';