title: Backbone: Check if model's attributes changed or at previous state
body: <p>I've a scenario where a user can update his/her profile. By default, the <strong>SAVE</strong> button is disabled and only gets enabled when a user changes any field. </p>
  
  <p>He has something like this:</p>
  
  <p><a href="https://i.stack.imgur.com/2sSsF.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/2sSsF.png" alt="enter image description here"></a></p>
  
  <p>Now if a user changes his/her first/last name, as:</p>
  
  <p><a href="https://i.stack.imgur.com/w5G15.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/w5G15.png" alt="enter image description here"></a></p>
  
  <p>The <strong>SAVE</strong> button is enabled and I maintain a status for any field change. What I'm doing here is:</p>
  
  <pre><code>firstNameChanged: function(){
              this.status = this.model.hasChanged("firstName");
              this.model.set("firstName",  this.$('#firstName').val());
          },
  lastNameChanged: function(){
              this.status = this.model.hasChanged("lastName");
              this.model.set("lastName",  this.$('#lastName').val());
          }
  </code></pre>
  
  <p>And I handle the model change scenario as:</p>
  
  <pre><code>this.model.on('change', this.enableSaveButton, this);
  </code></pre>
  
  <p>This works fine until the user changes his/her First/Last name, but when a user changes his/her names back to previous one, I want to <strong>DISABLE</strong> the <strong>SAVE</strong> button, that in my case I'm confused on how to go about it. Please suggest the right way to go about it. Thanks in advance!</p>