Check if Email is valid via JavaScript

/**
 * Check if given String is a valid Email address
 *
 * @param   string      An email address
 * @return  boolean
 */
function validEmail(s) {
    var a = false;
    var res = false;
    if(typeof(RegExp) == 'function') {
        var b = new RegExp('abc');
        if(b.test('abc') == true){a = true;}
    }
        if (a == true) {
        reg = new RegExp('^([a-zA-Z0-9\\-\\.\\_]+)' + '(\\@)([a-zA-Z0-9\\-\\.]+)'+ '(\\.)([a-zA-Z]{2,4})$');
        res = (reg.test(s));
    } else {
        res = (s.search('@') >= 1 && s.lastIndexOf('.') > s.search('@') && s.lastIndexOf('.') >= s.length-5);
    }
    return(res);
}