jquery ajax Uncaught TypeError: Illegal invocation
This error is due to formData which is json object and by default ajax requrie values to be in string format and as a solution 1. you can either remove formData and pass the values separately data: { username: document.getElementById('createUserForm').value } 2. you can set the dataType to be 'json' and processData to be false $.ajax({ url : base_url+'index.php', type: 'POST', dataType: 'json', data: data, cache : false, processData: false })