title: reactjs how to update state
body: <pre><code>constructor(){
      super();
      this.state = { 
          address: {
              street:null,
              city:null,
              postalCode: null
          }
      };
  }
  postalCodeChange(e){
      this.setState.address.postalCode = e.target.value;
      console.log(this.state);
  }
  </code></pre>
  
  <p>I'm trying to update the state, but the postalCode only. When the above code execute, I expect to get something like this</p>
  
  <pre><code>Object {address: Object {
             street: null,
             city: null,
             postalCode: 'some new value'
  } }
  </code></pre>
  
  <p>But I got error. What's wrong?</p>