title: Get template variable in onCreated in Meteor
body: <p>I am rendering a template with <code>Blaze.renderWithData(Template.templateName, { key: value })</code>;</p>
  
  <p>I can get the value in my template with <code>{{key}}</code>, but I cannot get the value in my js code.</p>
  
  <p>I have tried</p>
  
  <pre><code>Template.templateName.onCreated( () =&gt; {
    console.log(Template.instance().key);
  });
  </code></pre>
  
  <p>but the variable is undefined.</p>