javascript - Accessing global object from content script in chrome extension

Question

I have defined a global object in a .js file. For example file1.js contains the global object SomeObject. This file gets loaded in background.html.

Since file1.js is included in background.html, I am able to access the global object in this page. So no issues here.

When an event like click on the extension button is performed, I am running a content script using the chrome.tabs.executeScript(null, {file: "contentscript.js"}); api.

How can I access SomeObject in the contentscript in this case?

Answer 1

There is no way to get direct access to the background page's global object from a Content script or injected script.

To use a background page's method from an injected script , follow these steps:

1. Content Script: Bind an event listener to the page ^{example 1}.
   Whenever you want to notify the method from the background page:
2. Injected script: Create and fire this specific event ^{example 1}.
   → The event listener from 1) gets triggered.
3. In this event listener, use chrome.runtime.sendMessage to request the functionality from the background ^{example 2}.
4. In the background page, handle this request using chrome.runtime.onMessage.
5. Optionally, inject code in the original tab using chrome.tabs.executeScript(tab.id, func).

To use a background page's method from a Content script, only steps 3 and 4 are needed.
Here's an example, in which the content script communicates with the background page:

// Example background page
function some_method(arg_name) {
    return localStorage.getItem(arg_name);
}
chrome.runtime.onMessage.addListener(function(request, sender, callback) {
    if (request.type == 'localStorage - step 4') {
        callback( some_method(request.name) );
    } else if (request.type == 'localStorage - step 5') {
        localStorage.setItem(request.name, request.value);
    }
});

// Example contentscript.js
chrome.runtime.sendMessage({
    type: 'localStorage - step 4',
    name: 'preference'
}, function(value) {
    if (value === null) {
        // Example: If no preference is set, set one:
        chrome.runtime.sendMessage({
            type: 'localStorage - step 5',
            name: 'preference',
            value: 'default'
        });
    }
});

See also

• SO: What you should know about background scripts vs Content Scripts vs Injected scripts.
• SO: Example code: Communicating between an Injected script and a Content script.