title: I'd like to prevent componentWillUnmount like beforeunload in React
body: <p>I'd like to do beforeunload with React, but I cannot.
  For example</p>
  
  <pre><code>window.addEventListener("beforeunload", function (e) {
    var confirmationMessage = "\o/";
  
    e.returnValue = confirmationMessage;     // Gecko and Trident
    return confirmationMessage;              // Gecko and WebKit
  });
  </code></pre>
  
  <p>But, I wanna do as below</p>
  
  <pre><code>componentWillUnmount: function(e) {
      var result = confirm('\o/');
      if (!result) {
         // prevent unmount
      }
  },
  </code></pre>