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// Testeado:
// jQuery 2.2.3 - 3.1.1
// Bootstrap 3
function findBootstrapEnvironment() {
    var envs = ['xs', 'sm', 'md', 'lg'],
        $el = $('<div>'),
        i = 0,
        env;

    $el.appendTo($('body'));

    for (i = envs.length - 1; i >= 0; i--) {
        env = envs[i];

        $el.addClass('hidden-' + env);
        if ($el.is(':hidden')) {
            $el.remove();
            return env;
        }
    }
}

window.alert(findBootstrapEnvironment());