Dynamic class in Angular.js

If you just want to add a class, use the class attribute with interpolation:

class="priority-is-{{todo.priority}}"

You can simply assign a function as an expression and return proper class from there. Edit: there is also better solution for dynamic classes. Please see note below.

Snippet from view:

<div ng-class="appliedClass(myObj)">...</div>

and in the controller:

$scope.appliedClass = function(myObj) {
    if (myObj.someValue === "highPriority") {
        return "special-css-class";
    } else {
        return "default-class"; // Or even "", which won't add any additional classes to the element
    }
}

Better way of doing this

I've recently learned about another approach. You pass in an object which has properties corresponding to the classes you operate on, and the values are expressions or boolean variables. A simple example:

ng-class="{ active: user.id == activeId }"

In this case active class will be added to the element as long as user.id matches activeId from the $scope object!

You almost got it :)
It should be without interpolation markup ({{ and }}):

<li ng-repeat="todo in todos" ng-class="todo.priority">
...