undefined - default -- will toggle on mouse over

<div class = "popover" *gistPopover= "''" >

undefined

</div>

undefined
false

<div class= "popover" *gistPopover= "'isVisible:false;'" >

false

</div>

false
true

<div class= "popover" *gistPopover= "'isVisible:true;'" >

true

</div>

true