How do I toggle an ng-show in AngularJS based on a boolean?

Question

You just need to toggle the value of "isReplyFormOpen" on ng-click event

<a ng-click="isReplyFormOpen = !isReplyFormOpen">Reply</a>
 <div ng-show="isReplyFormOpen" id="replyForm">
   </div>

Basically I solved it by NOT-ing the isReplyFormOpen value whenever it is clicked:

<a ng-click="isReplyFormOpen = !isReplyFormOpen">Reply</a>

<div ng-init="isReplyFormOpen = false" ng-show="isReplyFormOpen" id="replyForm">
    <!-- Form -->
</div>

If based on click here it is:

ng-click="orderReverse = orderReverse ? false : true"

It's worth noting that if you have a button in Controller A and the element you want to show in Controller B, you may need to use dot notation to access the scope variable across controllers.

For example, this will not work:

<div ng-controller="ControllerA">
  <a ng-click="isReplyFormOpen = !isReplyFormOpen">Reply</a>

  <div ng-controller="ControllerB">
    <div ng-show="isReplyFormOpen" id="replyForm">
    </div>
  </div>

</div>

To solve this, create a global variable (ie. in Controller A or your main Controller):

.controller('ControllerA', function ($scope) {
  $scope.global = {};
}

Then add a 'global' prefix to your click and show variables:

<div ng-controller="ControllerA">
  <a ng-click="global.isReplyFormOpen = !global.isReplyFormOpen">Reply</a>

  <div ng-controller="ControllerB">
    <div ng-show="global.isReplyFormOpen" id="replyForm">
    </div>
  </div>

</div>

For more detail, check out the Nested States & Nested Views in the Angular-UI documentation, watch a video, or read understanding scopes.