var app = angular.module('plunker', []);

app.controller('MainCtrl', function($scope) {
  $scope.test = 'NO';
  $scope.test2 = 'NOO';
  
  $scope.testNo = 'no';
  $scope.testYes = 'yes';
  $scope.testOn = 'on';
  $scope.testOff = 'off';
});
<!DOCTYPE html>
<html ng-app="plunker">

  <head>
    <meta charset="utf-8" />
    <title>AngularJS Plunker</title>
    <script>document.write('<base href="' + document.location + '" />');</script>
    <link rel="stylesheet" href="style.css" />
    <script data-require="angular.js@1.2.x" src="https://code.angularjs.org/1.2.28/angular.js" data-semver="1.2.28"></script>
    <script src="app.js"></script>
    <style>
      .success{
        background: #0F0;
      }
      .error{
        background: #F00;
      }
    </style>
  </head>

  <body ng-controller="MainCtrl">
    <h1>O_o" ng-show</h1>
    1. if scope is string 'NOO':
    <span class='success' ng-show="test2">{{ test2 }}</span>
    <br>
    <br>
    
    2. if scope is string 'NO':
    <span class='success' ng-show="test">{{ test }}</span>
    <span class='error' ng-show="!test">!test</span>
    <br>
    ------------------------------------^ for now this value is not show ?!
    <br>
    <br>
    
    3. try $scope.test is true/false ?  
    <span ng-class = "test ? 'success' : 'error'">
      {{ test ? 'true' : 'false' }}
    </span>
    <br>
    <br>

    4. Solution:
    
    <pre>
      ng-show="test ? true : false"
    </pre>
    sample:
    <span class='success' ng-show="test ? true : false">{{ test }}</span>
    <br>
    <br>
    <hr>
    
    <h2>more test:</h2>
    test No:
    <div ng-show='testNo' class='success'>scope value = 'NO' ng-show</div>
    <div ng-hide='testNo' class='error'>scope value = 'NO' ng-hide</div>
    test Yes:
    <div ng-show='testYes' class='success'>scope value = 'YES' ng-show</div>
    <div ng-hide='testYes' class='error'>scope value = 'YES' ng-hide</div>
    test On:
    <div ng-show='testOn' class='success'>scope value = 'ON' ng-show</div>
    <div ng-hide='testOn' class='error'>scope value = 'ON' ng-hide</div>
    test Off:
    <div ng-show='testOff' class='success'>scope value = 'Off' ng-show</div>
    <div ng-hide='testOff' class='error'>scope value = 'Off' ng-hide</div>

  </body>

</html>
/* Put your css in here */