Override user agent on all browsers

setUserAgent.js

function setUserAgent(window, userAgent) {
    // Works on Firefox, Chrome, Opera and IE9+
    if (navigator.__defineGetter__) {
        navigator.__defineGetter__('userAgent', function () {
            return userAgent;
        });
    } else if (Object.defineProperty) {
        Object.defineProperty(navigator, 'userAgent', {
            get: function () {
                return userAgent;
            }
        });
    }
    // Works on Safari
    if (window.navigator.userAgent !== userAgent) {
        var userAgentProp = {
            get: function () {
                return userAgent;
            }
        };
        try {
            Object.defineProperty(window.navigator, 'userAgent', userAgentProp);
        } catch (e) {
            window.navigator = Object.create(navigator, {
                userAgent: userAgentProp
            });
        }
    }
}

Since long time ago browsers have prohibited spoofing User-Agent header in http requests.
No matter what you do, the browse still gonna send the real UA header.
Two methods to overcome that situation:

1. Use browser extension, that modifies outgoing http request headers (webRequest persmission)
2. Send the request from a server