What angularjs expression syntax is this in ng-class

This is mentioned briefly (too briefly, in my opinion) in the ngClass documentation. If you pass an object to ngClass, then it will apply each key of the object as a class to the element if that key’s value is true. For example:

$scope.first = true
$scope.second = false
$scope.third = true

with

<div ng-class="{a: first, b: second, c: third}"></div>

would result in

<div class="a c"></div>