title: Why is react returning a TypeError: Cannot read properly 'join' of undefined?
body: <p>I need to display a list of books, and the name of the author(s) below.</p>
  
  <p>In case there are multiple authors, I need to separate their names and I'm trying to use .join().</p>
  
  <p>Below part of the code without .join():</p>
  
  <pre><code>
  &lt;select
    onChange={(event) =&gt; this.props.changeShelf(
     this.props.book, event.target.value
    )}
    value={this.props.actualShelf}
  &gt;
    &lt;option value='move' disabled&gt;Move to...&lt;/option&gt;
    &lt;option value='currentlyReading'&gt;Currently Reading&lt;/option&gt;
    &lt;option value='wantToRead'&gt;Want to Read&lt;/option&gt;
    &lt;option value='read'&gt;Read&lt;/option&gt;
    &lt;option value='none'&gt;None&lt;/option&gt;
  &lt;/select&gt;
  &lt;/div&gt;
    &lt;/div&gt;
      &lt;div className='book-title'&gt;{this.props.book.title}&lt;/div&gt;
      &lt;div className='book-authors'&gt;{this.props.book.authors}&lt;/div&gt;                  
    &lt;/div&gt;            
  );
  
  
  </code></pre>
  
  <p>And next the code with .join():</p>
  
  <pre><code>
  &lt;select
    onChange={(event) =&gt; this.props.changeShelf(
     this.props.book, event.target.value
    )}
    value={this.props.actualShelf}
  &gt;
    &lt;option value='move' disabled&gt;Move to...&lt;/option&gt;
    &lt;option value='currentlyReading'&gt;Currently Reading&lt;/option&gt;
    &lt;option value='wantToRead'&gt;Want to Read&lt;/option&gt;
    &lt;option value='read'&gt;Read&lt;/option&gt;
    &lt;option value='none'&gt;None&lt;/option&gt;
  &lt;/select&gt;
  &lt;/div&gt;
    &lt;/div&gt;
      &lt;div className='book-title'&gt;{this.props.book.title}&lt;/div&gt;
      &lt;div className='book-authors'&gt;{this.props.book.authors.join(', ')}&lt;/div&gt;                  
    &lt;/div&gt;            
  );
  
  </code></pre>
  
  <p>My issue is that if I don't use .join(), the page works well, but when I insert .join(), initially the page works, but when I go to the search page and try to look for a book, i get the following error: <code>"TypeError: Cannot read properly 'join' of undefined"</code>. What could I do to solve this issue?</p>
  
  <p>Obs. 1: I didn't put the entire code, but if you need to see it from the beginning, please let me know and I will edit this post.</p>
  
  <p>Obs. 2: This component isn't a Stateless Component but a Class Component, that's why if you need the entire component displayed here, please let me know.</p>