jquery ajax uncaught typeerror 3a illegal invocation

1This error is due to formData which is json object and by default ajax requrie
2values to be in string format and as a solution
3
41. you can either remove formData and pass the values separately 
5	data: { username: document.getElementById('createUserForm').value }
6
72. you can set the dataType to be 'json' and processData to be false
8    
9	$.ajax({
10        url : base_url+'index.php',
11        type: 'POST',
12        dataType: 'json',
13        data: data,
14        cache : false,
15        processData: false
16    })