title: Use value if it exists, else assign default using the or operator
body: <p>I found this example in a book:</p>
  
  <pre><code>// Create _callbacks object, unless it already exists
  var calls = this._callbacks || (this._callbacks = {});
  </code></pre>
  
  <p>I simplified it so that I did not have to use a special object scope:</p>
  
  <pre><code>var a = b || (b = "Hello!");
  </code></pre>
  
  <p>When b is defined, it works. When b is not defined, it does not work and throws a ReferenceError.</p>
  
  <pre><code>ReferenceError: b is not defined
  </code></pre>
  
  <p>Did I do anything wrong? Thank you!</p>