when do we get TypeError: Illegal invocation error code example

Example: TypeError: Illegal invocation

Your datatype is not JSON, it’s a FormData. And for jQuery to send a FormData, it needs…
$.ajax({
    type: "POST",
    url: 'signin2.php',
    data: formData,
    processData: false, //add this
    contentType: false, //and this
})