[aliveupstairs]

the docs:

https://vuejs.org/v2/guide/class-and-style.html#Binding-HTML...

https://vuejs.org/v2/guide/class-and-style.html#Array-Syntax

In your template:

a) <div :class={active: isActive} />

b) <div :class="{isActive? 'active' : 'inactive' }"/>

[earthboundkid]

B) is not valid. It needs to be wrapped in an Array, not an Object.