How to disable bootstrap button after one click using javascript

How do I disable a Bootstrap button after it is clicked using Javascript. I am using the onclick event but it doesn't get disabled.

Code:

<div class="panel-heading">
    <div class="btn-group pull-right"> 
         <a href="/assign" class="btn btn-success">Assign</a>
         <a href="#" class="btn btn-primary" onclick="this.disabled=true">Upload</a>
   </div>
</div>