Cross-domain XMLHttpRequest using background pages

You don’t have to mess with iframes. It’s possible to perform cross-domain XMLHttpRequests, using background pages. Since Chrome 13, cross-site requests can be made from the content script. However, requests can still fail if the page is served with a Content Security Policy header with a restricting connect-src.

Another reason for choosing the nexy method over content scripts is that requests to http sites will cause a mixed content warning (“The page at https://… displayed insecure content from http://…”).

Yet another reason for delegating the request to the background page is when you want to get a resource from the file://, because a content script cannot read from file:, unless it is running on a page at the file:// scheme.

Note
To enable cross-origin requests, you have to explicitly grant permissions to your extension using the permissions array in your manifest file.

Cross-site request using background script.

The content script would request the functionality from the background via the messaging API. Here is an example of a very simple way of sending and getting the response of a request.

chrome.runtime.sendMessage({
    method: 'POST',
    action: 'xhttp',
    url: 'http://www.stackoverflow.com/search',
    data: 'q=something'
}, function(responseText) {
    alert(responseText);
    /*Callback function to deal with the response*/
});

Background / event page:

/**
 * Possible parameters for request:
 *  action: "xhttp" for a cross-origin HTTP request
 *  method: Default "GET"
 *  url   : required, but not validated
 *  data  : data to send in a POST request
 *
 * The callback function is called upon completion of the request */
chrome.runtime.onMessage.addListener(function(request, sender, callback) {
    if (request.action == "xhttp") {
        var xhttp = new XMLHttpRequest();
        var method = request.method ? request.method.toUpperCase() : 'GET';

        xhttp.onload = function() {
            callback(xhttp.responseText);
        };
        xhttp.onerror = function() {
            // Do whatever you want on error. Don't forget to invoke the
            // callback to clean up the communication port.
            callback();
        };
        xhttp.open(method, request.url, true);
        if (method == 'POST') {
            xhttp.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
        }
        xhttp.send(request.data);
        return true; // prevents the callback from being called too early on return
    }
});

Remark: The messaging APIs have been renamed several times. If your target browser is not the latest Chrome version, check out this answer.

For completeness, here’s a manifest file to try out my demo:

{
    "name": "X-domain test",
    "manifest_version": 2,
    "permissions": [
        "http://www.stackoverflow.com/search*"
    ],
    "content_scripts": {
        "js": ["contentscript.js"],
        "matches": ["http://www.example.com/*"]
    },
    "background": {
        "scripts": ["background.js"],
        "persistent": false
    }
}