Chrome extension - Write to local storage of a different domain

Question

How can I write to a local storage of a different domain. The idea is that I need my chrome extension to write something in the local storage and when the user visits the associated website, site site can read the contents of the local storage. I am trying to do a sync between the user's personal data without having to store them in the server.

Answer 1

Content scripts have direct acccess to a page's localStorage.

If you want to store a value for a specific domain, just open a window or frame, then write to the window/page's local storage.

1. Pick your favorite option to activate the page:

• chrome.tabs.create to create an inactive tab, perhaps in the non-active window found by using chrome.tabs.query.
• The experimental offscreenTabs API (experimental, so not ready for use in production). See some of the examples I posted on SO.
• Create an IFrame and insert it in the current page.
• window.open (not recommended...)

When you decide to open a page, pick one which is light-weight. /robots.txt is usually a good choice: It exists on most sites, and it is a plain text file.

2. Activate the content script for the page. You can either use the manifest file and make use of the messaging APIs, or add a callback to the chrome.tabs.create method which programatically inserts a content script (chrome.tabs.executeScript).

Here's a simple example. It requires the tabs API because I'm using chrome.tabs.executeScript. Furthermore, the origin you want to access should also be added to the permissions list.

chrome.tabs.create({
    active: false,
    url: 'http://stackoveflow.com/robots.txt'
}, function(tab) {
    chrome.tabs.executeScript(tab.id, {
        code: 'localStorage.setItem("key", "value");'
    }, function() {
        chrome.tabs.remove(tab.id);
    });
});

---

Update (2022)

Since Manifest version 3 the chrome.tabs.executeScript API was replaced with the chrome.scripting.executeScript API, hence the code above can be changed to be like this:

function saveToLocalStorage() {
    localStorage.setItem("key", "value");
}

chrome.tabs.create({
    active: false,
    url: 'http://stackoveflow.com/robots.txt'
}, function(tab) {
    chrome.scripting.executeScript({
        target: {tabId: tab.id},
        func: saveToLocalStorage,
    }, function() {
        chrome.tabs.remove(tab.id);
    });
});

And please note, you need to add the "scripting" permission to your manifest.json file in order to use this API.

Answer 2

I think the only solution to do this is via script injection due to cross domain restrictions.

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

Add a file script.js to your extension accessing the other sites localStorage, you have to add script.js to "web_accessible_resources" in your manifest file.

Note that the script is automatically removed after the script was completely loaded, so make sure that the code you want to execute is contained in a self executable function.