What is the best way to conditionally apply a class?

If you don't want to put CSS class names into Controller like I do, here is an old trick that I use since pre-v1 days. We can write an expression that evaluates directly to a class name selected, no custom directives are necessary:

ng:class="{true:'selected', false:''}[$index==selectedIndex]"

Please note the old syntax with colon.

There is also a new better way of applying classes conditionally, like:

ng-class="{selected: $index==selectedIndex}"

Angular now supports expressions that return an object. Each property (name) of this object is now considered as a class name and is applied depending on its value.

However these ways are not functionally equal. Here is an example:

ng-class="{admin:'enabled', moderator:'disabled', '':'hidden'}[user.role]"

We could therefore reuse existing CSS classes by basically mapping a model property to a class name and at the same time keep CSS classes out of Controller code.

ng-class supports an expression that must evaluate to either

1. A string of space-delimited class names, or
2. An array of class names, or
3. A map/object of class names to boolean values.

So, using form 3) we can simply write

ng-class="{'selected': $index==selectedIndex}"

See also How do I conditionally apply CSS styles in AngularJS? for a broader answer.

Update: Angular 1.1.5 has added support for a ternary operator, so if that construct is more familiar to you:

ng-class="($index==selectedIndex) ? 'selected' : ''"

My favorite method is using the ternary expression.

ng-class="condition ? 'trueClass' : 'falseClass'"

Note: Incase you're using a older version of Angular you should use this instead,

ng-class="condition && 'trueClass' || 'falseClass'"