title: Angular [class.active]="isActive" - what does "class.active" mean here?
body: <p>In a training video of angular, I saw some codes, like these below:</p>
  
  <p><code>&lt;button class="nav nav-primary" [class.active]="isActive"&gt;</code></p>
  
  <p>and in the component, If we let the variable <code>isActive=true</code>,then the button has the  "active" class;if let the variable <code>isActive=false</code>,then the button has not the "active" class.</p>
  
  <p>I know that this is a one-way binding, but I want to know what's the grammar mean of "class.active"? (the "class" has a property of"active"? but this is impossible.</p>