title: Why the console output undefined?
body: <p>I have code following like this:</p>
  
  <pre><code>function Person() {
      this.name = '123';
      this.age = 123;
  }
  Person.prototype.load = function() {
      console.log(this.name + " test "); 
  }
  var p1 = new Person();
  console.log(p1.load());
  </code></pre>
  
  <p>the console output two news. One is <code>123 test</code>, the other is <code>undefined</code>.I wonder where is  <code>undefined</code> comes from?</p>