Changing User-Agent in XMLHttpRequest from a Chrome extension

Question

I'm trying to send a HTTP request from a Extension in which I need to change the User-Agent.

My code looks like this:

function getXMLHttpRequest(method, url, extraHeaders) {
    var xhr = new XMLHttpRequest();
    xhr.open(method, url, true)

    for (var headerKey in extraHeaders) {
        xhr.setRequestHeader(headerKey, extraHeaders[headerKey]);
    }

    return xhr;
}
//....

getXMLHttpRequest("POST", "....", { "User-Agent": "Blahblahblah" })

Then, I get an error "Refused to set unsafe header: UserAgent"

I need to change that because my Backend needs to have an special User-Agent, is it possible to do that from an extension?

I tried webRequest API, to change the header before sending the request, but it says it does not work with XMLHttpRequest made from extensions in order to prevent locking.

Answer 1

You can easily change the User-Agent header with the webRequest API.
For sample code, see Associate a custom user agent to a specific Google Chrome page/tab.

Take the code from that answer, and change "main_frame", "sub_frame" to "xmlhttprequest" to modify network requests initiated via XMLHttpRequest.

Obviously, to prevent deadlocks, this method does not work with synchronous requests ( i.e. when the third parameter of xhr.open is set to false).