JQuery .hasClass for multiple values in an if statement

You just had some messed up parentheses in your 2nd attempt.

var $html = $("html");

if ($html.hasClass('m320') || $html.hasClass('m768')) {

  // do stuff 

}

You could use is() instead of hasClass():

if ($('html').is('.m320, .m768')) { ... }

For fun, I wrote a little jQuery add-on method that will check for any one of multiple class names:

$.fn.hasAnyClass = function() {
    for (var i = 0; i < arguments.length; i++) {
        if (this.hasClass(arguments[i])) {
            return true;
        }
    }
    return false;
}

Then, in your example, you could use this:

if ($('html').hasAnyClass('m320', 'm768')) {

// do stuff 

}

You can pass as many class names as you want.

---

Here's an enhanced version that also lets you pass multiple class names separated by a space:

$.fn.hasAnyClass = function() {
    for (var i = 0; i < arguments.length; i++) {
        var classes = arguments[i].split(" ");
        for (var j = 0; j < classes.length; j++) {
            if (this.hasClass(classes[j])) {
                return true;
            }
        }
    }
    return false;
}

if ($('html').hasAnyClass('m320 m768')) {
    // do stuff 
}

Working demo: http://jsfiddle.net/jfriend00/uvtSA/

This may be another solution:

if ($('html').attr('class').match(/m320|m768/)) {  
  // do stuff   
}  

according to jsperf.com it's quite fast, too.