title: Uncaught TypeError: Cannot read property 'getContext' of null
body: <p>In my console I get the error: "Uncaught TypeError: Cannot read property 'getContext' of null" 
  and I just can't find the error I've made... or what I've done wrong. 
  So maybe you can help me find it? 
  Please help :) </p>
  
  <pre><code>enter code here
  
  var canvas = document.getElementById("myCanvas");
  
  var ctx = canvas.getContext("2d");
  
  var cW = canvas.width = 1000; 
  var cH = canvas.height = 500; 
  
  var particleAmount = 10; 
  var particles = []; 
  
  for(var i=0;i&lt;particleAmount;i++) { 
  particles.push(new particle());
  
  }
  
  function particle() { 
  this.x = (Math.random() * (cW-(40*2))) + 40; 
  this.y = (Math.random() * (cH-(40*2))) + 40; 
  this.xv = Math.random()*20-10; 
  this.yv = Math.random()*20-10; 
  
  }
  
  function draw () { 
  ctx.fillStyle = "black";
  ctx.fillRect(0,0,cW,cH);
  
  for(var ii=0;ii&lt;particles.length;ii++){
      var p = particles[ii]; 
      ctx.fillStyle = "red";
      ctx.beginPath(); 
      ctx.arc(p.x,p.y,40,Math.PI*2,false); 
      ctx.fill();
  }
  
  
  }
  
  setInterval(draw,30);
  </code></pre>