Uncaught Reference error $ is not defined

[eluser]JasmineFlower[/eluser]
hi,


This my javascript file. It display Uncaught Reference error $ is not defined.

Code:

function checkavailableNames()
{        
   var username=document.registration_form.indiv_reg_loginname.value
   if(username == '')
   {
      document.getElementById('indiv_username_avail_check')[removed]="Required field cannot be left blank";
      document.registration_form.indiv_reg_loginname.focus();
      return false;
    }
    else
    {    
    $.ajax({
    type: 'POST',
    dataType: 'json',
    url: base_url + 'register/checkIndivUsernameAvail/' + Math.random(),
    data: 'username=' + username,
    success: function(response)
        {            
       var availcontent    = response.result;
       if(availcontent.length > 0)
       {
            document.getElementById('indiv_username_avail_check')[removed]="User name is not available";
        return false;
       }
       else
       {
        document.getElementById('indiv_username_avail_check')[removed]="User name is available";
        return true;
       }                                    
    }                
    });            
    }    
}

Pls anyone tell me how to solve this?

[eluser]predat0r[/eluser]
Try to put jquery reference first in the html header.