This angularjs directive adds/removes a class if an expression is true. It requires jQuery and uses the expression syntax "(`expression`) ? `className` [: `className`]".

gistfile1.js

angular.directive("ng:class", function (expression, compiledElement) {
	if (typeof(expression) == "string") {
		if (expression.indexOf(",")) {
			expression = expression.split(",");
		} else {
			expression = [expression];
		}
	}
	
	return function (element) {
		for (var i = 0; i < expression.length; i ++) {
			expression[i] = expression[i].replace(" ", "");
			var className = expression[i].substring(expression[i].indexOf(")?") + 2);
			var elseClass = "";
			if (className.indexOf(":") > 0) {
				className = className.split(":");
				elseClass = className.pop();
				className = className.pop();
			}
			var watch = expression[i].substring(expression[i].indexOf("(") + 1, expression[i].indexOf(")"));
			
			(function (self, className, watch) {
				self.$watch(watch, function (value) {
					if (value) $(element).addClass(className).removeClass(elseClass);
					else $(element).removeClass(className).addClass(elseClass);
				});
			})(this, className, watch);
		}
	}
});