title: clearRect function doesn't clear the canvas
body: <p>I'm using this script on the body <code>onmousemove</code> function:</p>
  
  <pre><code>function lineDraw() {
      // Get the context and the canvas:
      var canvas = document.getElementById("myCanvas");
      var context = canvas.getContext("2d");
      // Clear the last canvas
      context.clearRect(0, 0, canvas.width, canvas.height);
      // Draw the line:
      context.moveTo(0, 0);
      context.lineTo(event.clientX, event.clientY);
      context.stroke();
  }
  </code></pre>
  
  <p>It's supposed to clear the canvas each time I move the mouse around, and draw a new line, but it isn't working properly. 
  I'm trying to solve it without using jQuery, mouse listeners or similar.</p>
  
  <p>Here is a demo: <a href="https://jsfiddle.net/0y4wf31k/" rel="noreferrer">https://jsfiddle.net/0y4wf31k/</a></p>