What is the best way to conditionally apply a class?

Question

Lets say you have an array that is rendered in a ul with an li for each element and a property on the controller called selectedIndex.

Answer 1

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.

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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' : ''"