title: Programmatically use RGBa values in fillStyle in <canvas>?
body: <p>Using <code>&lt;canvas&gt;</code>, I want to set the RGBa value of the rectangle using a variable. </p>
  
  <p>for example:</p>
  
  <pre><code>ctx.fillStyle = "rgba(32, 45, 21, 0.3)";
  </code></pre>
  
  <p>works fine, but using it with a variable:</p>
  
  <pre><code>var r_a =  0.3;
  ctx.fillStyle = "rgba(32, 45, 21, r_a)";
  </code></pre>
  
  <p>doesn't work.</p>
  
  <p>Apparently <code>fillStyle</code> only accepts a string. So how do I set a value of the rgba value using some variable instead of explicitly defining the values?</p>