XMLHttpRequest проверка файла

function request(url, callback)
{
    var xhr = new XMLHttpRequest();
    xhr.onreadystatechange = (function(myxhr)
    {
        return function()
        {
            if(myxhr.readyState === 4) callback(myxhr);
        }
    })(xhr);
    xhr.open('GET', url, true);
    xhr.send('');
}

request("./camera-resolutions.json", function(o)
{
...
}

function ifFileExist(url)
{
    var xhr = new XMLHttpRequest();
    xhr.open('GET', url, true);
    xhr.send('');
    xhr.onreadystatechange = function()
    {
        if(xhr.readyState === 4)
        {
            return xhr.status == 200 ? true : false;
        }
    }
}

var settings_file = ifFileExist(settingsFile) ? settingsFile : "./camera-resolutions.json";
request(settings_file, function(o)
{
...
}