title: what's the different among a,window.a,this.a?
body: <pre><code>a1=10;
  d1=new Date();
  for( var i=0;i&lt;=100000;i++){
   a1;
  }
  console.log(new Date()-d1,a1===window.a1);
  
  d2=new Date();
  for( var i=0;i&lt;=100000;i++){
   this.a1;
  }
  console.log(new Date()-d2,this.a1===window.a1);
  
  d3=new Date();
  for( var i=0;i&lt;=100000;i++){
   window.a1;
  }
  console.log(new Date()-d3,this.a1===window.a1);
  </code></pre>
  
  <p>//output<br>
  116,true</p>
  
  <p>53,true</p>
  
  <p>98,true</p>
  
  <p>In the global,a1===this.a===window.a,why the speed is so different?</p>
  
  <p>(In chrome 39.0.2171.71 m)</p>