title: Uncaught TypeError: undefined is not a function
body: <p>I am newbie to js, I have copied a script from jqueryui.com for the dialog widget and pasted it in my Yii project, but I got an error: Uncaught TypeError: undefined is not a function, which is the function of <code>$( "#dialog-form" ).dialog</code>, so what is the error, and how I can fix it, here is the code:</p>
  
  <pre><code>$(function() {
      var name = $( "#name" ),
        email = $( "#email" ),
        password = $( "#password" ),
        allFields = $( [] ).add( name ).add( email ).add( password ),
        tips = $( ".validateTips" );
  
      function updateTips( t ) {
        tips
          .text( t )
          .addClass( "ui-state-highlight" );
        setTimeout(function() {
          tips.removeClass( "ui-state-highlight", 1500 );
        }, 500 );
      }
  
      function checkLength( o, n, min, max ) {
        if ( o.val().length &gt; max || o.val().length &lt; min ) {
          o.addClass( "ui-state-error" );
          updateTips( "Length of " + n + " must be between " +
            min + " and " + max + "." );
          return false;
        } else {
          return true;
        }
      }
  
      function checkRegexp( o, regexp, n ) {
        if ( !( regexp.test( o.val() ) ) ) {
          o.addClass( "ui-state-error" );
          updateTips( n );
          return false;
        } else {
          return true;
        }
      }
  
      $( "#dialog-form" ).dialog({
        autoOpen: false,
        height: 300,
        width: 350,
        modal: true,
        buttons: {
          "Create an account": function() {
            var bValid = true;
            allFields.removeClass( "ui-state-error" );
  
            bValid = bValid &amp;&amp; checkLength( name, "username", 3, 16 );
            bValid = bValid &amp;&amp; checkLength( email, "email", 6, 80 );
            bValid = bValid &amp;&amp; checkLength( password, "password", 5, 16 );
  
            bValid = bValid &amp;&amp; checkRegexp( name, /^[a-z]([0-9a-z_])+$/i, "Username may consist of a-z, 0-9, underscores, begin with a letter." );
            // From jquery.validate.js (by joern), contributed by Scott Gonzalez: http://projects.scottsplayground.com/email_address_validation/
            bValid = bValid &amp;&amp; checkRegexp( email, /^((([a-z]|\d|[!#\$%&amp;'\*\+\-\/=\?\^_`{\|}~]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])+(\.([a-z]|\d|[!#\$%&amp;'\*\+\-\/=\?\^_`{\|}~]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])+)*)|((\x22)((((\x20|\x09)*(\x0d\x0a))?(\x20|\x09)+)?(([\x01-\x08\x0b\x0c\x0e-\x1f\x7f]|\x21|[\x23-\x5b]|[\x5d-\x7e]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])|(\\([\x01-\x09\x0b\x0c\x0d-\x7f]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF]))))*(((\x20|\x09)*(\x0d\x0a))?(\x20|\x09)+)?(\x22)))@((([a-z]|\d|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])|(([a-z]|\d|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])([a-z]|\d|-|\.|_|~|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])*([a-z]|\d|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])))\.)+(([a-z]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])|(([a-z]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])([a-z]|\d|-|\.|_|~|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])*([a-z]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])))\.?$/i, "eg. ui@jquery.com" );
            bValid = bValid &amp;&amp; checkRegexp( password, /^([0-9a-zA-Z])+$/, "Password field only allow : a-z 0-9" );
  
            if ( bValid ) {
              $( "#users tbody" ).append( "&lt;tr&gt;" +
                "&lt;td&gt;" + name.val() + "&lt;/td&gt;" +
                "&lt;td&gt;" + email.val() + "&lt;/td&gt;" +
                "&lt;td&gt;" + password.val() + "&lt;/td&gt;" +
              "&lt;/tr&gt;" );
              $( this ).dialog( "close" );
            }
          },
          Cancel: function() {
            $( this ).dialog( "close" );
          }
        },
        close: function() {
          allFields.val( "" ).removeClass( "ui-state-error" );
        }
      });
  
      $( "#create-user" )
        .button()
        .click(function() {
          $( "#dialog-form" ).dialog( "open" );
        });
    });
  </code></pre>