gistfile1.js

function getFilterOpts(type, item, callback) {	
	var requestUrl = "/api/plugin:"+type+"/config?path=" +item;
	var str = '';
	
	// get json data from config
	glow.net.get(requestUrl, {
		onLoad: function(response) {								
			var data = response.json();
			str += '<p>'+data.a_value+'</p>';
			// simplified example of what I need to add to var str.
			// now call the callback
			callback(str);
		},
		onError: function(response) {
			alert("Error getting file: " + response.statusText());
			// you may want to replace the above with...
			callback('Argh, it all went wrong');
		},
		useCache: true
	});
	
	// you can't return str here as it hasn't been created yet
}


// usage:
getFilterOpts('blah', 'blah', function(str) {
	console.log(str);
});

// because your callback is async, this next line will run before the callback
console.log('This happens before the callback');

// when the callback runs is dependant on how long the server takes to respond