title: How to use canvas in Angular
body: <p>The common approaching to use canvas in javascript is like :</p>
  
  <pre><code>var canvas = document.getElementById('tutorial');
  var ctx = canvas.getContext('2d');
  </code></pre>
  
  <p>but in Angular2 I cannot get the HTMLCanvasElement object, the var "canvas" only get the HTMLElement in Angular2. So how to use canvas in Angular2?
  And furthermore, how to use the third-party javascript in Angular2 with the language TypeScript?</p>