title: Anonymous Function Error: $ is not defined
body: <p>Google Developer Tools is displaying the following error when my PHP page uses the content from a javascript file (my_scripts.js):</p>
  
  <p>"Uncaught ReferenceError: $ is not defined scripts.js:1 (anonymous function)"</p>
  
  <p>Content of my_scripts.js</p>
  
  <pre><code>$('a.button').click(function(){ 
      window.location.href = "another_page.php";         
  });
  </code></pre>
  
  <p>The page and the script work as required. Clicking the element links to the requested page, but the error is there.</p>
  
  <p>1) What causes the error?
  2) Can it or should it be ignored?</p>