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
})